صور الصفحة
PDF

some very small trilobites. Besides these, there are two species of a genus which has generally been considered identical with goniatites, although it appears to depart very widely from the type of that curious and well-known group.

In the Annales des Sciences Naturelles, for 1834*, will be found the translation of a paper by Count Münster, announcing the discovery of a new genus, which he calls Clymenia, and which he found among several new species of goniatites, in the transition limestone of the Fichtelgebirget. It is to this genus, hitherto unknown in English Palaeontology, that the newly-discovered Cornish fossils must be referred; although there appears to be so much difference in some respects,

that they may possibly form a sub-genus, peculiar to the formation in which they occur.

The name Clymenia, however, is peculiarly unfortunate, both because it is already appropriated, (for it designates one of Cuvier's genera of Annelides,) and also from its entire want of analogy with all other names of fossil cephalopods. As it must be abandoned, I would propose to call the genus Endosiphonites, which has the advantage of indicating the most remarkable and important character—the ventral position of the siphuncle; while at the same time it sufficiently resembles the names of allied genera, and by a slight alteration of the first two syllables, might be applied to mark the different positions of the siphuncle which characterise ammonites, nautilites, &c.

The peculiar character of this genus is, as I have already remarked, the position of the siphuncle, in which it differs both from ammonite

* When my paper was read before the Society in February, I was not aware of the existence of this notice in the Annales des Sciences. It is to Professor Phillips that I am indebted for the reference ; and he has already made use of it in the article “Goniatite,” in the Penny Cyclopedia.

t These hills are situated in the South-eastern part of Germany, to the East of the

Maine, and not very far North of Nuremberg. The central ridge is of granite and the transition limestone.

and nautilus. According to Count Münster, the species from the Fichtelgebirge admit of the following description:-The narrow siphuncle is constantly found on the ventral part of the spiral shell, where it passes through a succession of small funnel-shaped apertures in the chambers. The whorls of the spire are free, never entirely enveloping the inner ones; and the last, and part of the last but one, of the turns have no septa. The intersections of the septa with the shell form undulations, or simple lateral lobes, at oblique angles, and dorsal and lateral rounded saddles”; but the line of intersection is not denticulated as in goniatites, or marked in the more intricate lines which characterise the ammonite. The siphuncle not being generally visible, it is by means of the dorsal saddle that this new genus is distinguished from goniatite, which always has a dorsal lobe on the medial line of the back. Count Münster elsewhere observes, that it is so difficult to obtain specimens having the septa apparent, that without extreme care it is almost impossible to avoid error; and that it is still more rare to find the siphuncle visible, since in the new genus, as well as in goniatites, it is so slender and so close to the shell as to be usually invisible, even when the marble in which it is found is polished.

Now the condition of the Cornish specimens I have examined is very different from that of the German ones, and much more perfect in some respects than these seem to have been ; but there are many points in the above description which do not at all agree with my observations. One of the most important of these is the nature of the siphuncle, which seems to be obscure in Count Münster's species, but is very prominent and easily seen in those which I have made out. But it is not only easily seen—it is decidedly large; and although near the shell cannot possibly be overlooked. In one species the . diameter of the aperture in the septum is one-fifth of the extreme length of the septum ; a proportion much larger than is commonly found in any species of nautilus, and which indeed is only paralleled in a

* The word saddle is here used to denote those separations between the lobes upon which the mantle of the animal is supposed to have rested. Dr Buckland has explained the language introduced by Von Buch on this subject, in a note, page 353, of his Bridgewater Treatise, to which I must refer for a more complete explanation.

few species of orthoceratites. If then, as there seems every reason to suppose, the siphuncle is the most important character in the shells of multilocular cephalopods, this very great difference would of itself warrant the formation of a separate group. All the species from Cornwall are provided with decidedly large apertures in the chambers, and in all, these funnel-shaped tubes are easily seen, produced beyond the septum about half way into the next chamber.

But again, the markings on the shell which seem so useful in determining Count Münster with regard to any doubtful cases, are in our English species apparently not to be depended on. Our fossils are in beautiful condition; the actual shell certainly remains in one specimen at least, and we can trace a succession of transverse striae marked with great beauty and regularity upon it; but although the casts of the chambers may be separately examined, the nature and use of the lobes does not quite appear. One thing is certain, they do not correspond to the intersection of the septa and the shell, and in only one of three species do they occur at all. Some idea of the form of a septum will be obtained from Fig. 4, Plate VIII. which represents a side and front view of the cast of a chamber belonging to a species not determined.

The technical description of the genus will be thus expressed:—a discoid spiral multilocular shell; sides nearly simple; whorls contiguous, the last not enveloping the rest. Septa transverse, numerous, concave outwardly, and perforated on the ventral margin for a siphuncle.

In order to determine the place of this genus among other shells of cephalopods, it will be necessary to pay most attention to goniatite and nautilus, as it is to these that the nearest approximation is made. Von Buch gives as the character of the former group, the dorsal siphuncle of the ammonite, comparatively small and delicate; the lobes of the septa completely deprived of lateral denticulations, and the striae of growth resembling those of nautilus, in not being directed forwards, as in ammonites, but reflected backwards. The nautilus is known by its usually central and comparatively large siphuncle, and the greater or less envelopement of the whorls of the spire by the last one formed.

[ocr errors][ocr errors][ocr errors][ocr errors][ocr errors][graphic]
[ocr errors][ocr errors]
« السابقةمتابعة »