186 PERMIAN FOSSILS. 



shells — the Aasinus angulatus of the London Clay, and the A. obscurus of the Magnesian 

 Limestone. Both species agree remarkably well in their marginal outline — one side or 

 extremity being rounded, and the opposite side somewhat pointed : in other words, the 

 cardinal margin on one side of the umbones is convex, and, on the other side, sloping, 

 or somewhat concave : in a more essential point, however, they differ so completely as 

 to be referable, not only to different genera, but to different families. Axinus angulatus 

 has the cartilage placed on the rounded side of the umbone ; whereas in^. obscurus it 

 is placed on the sloping side. It follows, from this difference, that the former shell is 

 rounded behind, and acuminated in front ; and that the latter is rounded in front, and 

 acuminated behind : in short, in these respects (and some others which it is unnecessary 

 to notice in the present place), Axinus angulatus possesses the characters of certain 

 Lucinidce ; whereas A. obscurus agrees with Trigonia. Now, as Mr. J. Sowerby dis- 

 tinctly stated, that he considered the London Clay Axinus angulatus to be '^ the type 

 of the genus,"i it follows that the so-called Axinus obscurus must be removed to some 

 other generic group. It is herein placed in the present genus. 



ScJiizodus in many respects appears to be closely related to Myophoria, Bronn. 

 The dental system, for example, is apparently but slightly modified in each genus, 

 judging from the figure which Goldfuss and Bronn have published of the teeth of the 

 left valve of Myophoria Goldfussi. The thick posterior tooth of this species may 

 be supposed to be the homologue of the bifid tooth of the corresponding valve of 

 ScJdzodus truncatus. The same agreement may be asserted of the anterior tooth of 

 both shells. As I am not acquainted with any published figure of the teeth of the 

 right valve of Myophoria, it is impossible for me to proceed any further with the 

 comparison. 



As regards Trigonia, however, a more detailed comparison may be instituted. 

 Notwithstanding the striking difference, apparent at first sight, there is a remarkable 

 agreement between the dental system of the genus named and that of Schizodus. If 

 we view the teeth of Trigonia as largely developed examples of their kind, and in the 

 Ught in which they are described by Agassiz, not Lamarck and others, without attending 



^ Mr. Morris appears to have been the first to suggest that Axinus angulatus belonged to Turton's genus 

 Cryptoddn (vide Catalogue of British Fossils, p. 80) ; probably on account of its close resemblance to 

 Cryptodon Jlexuosus, Montague, -which is by some considered to be a species of Lucina. Considering the 

 type ot Lucina (viz. Venus Jamaicensis, Chem.), Montague's species evidently belongs to a distinct genus : 

 if this be admitted, the name Axinus will have to be applied to it in preference to Cryptodon ; as the former 

 had a few months' priority, being published in the ' Mineral Conchology' No. 55, Dec. 1821 ; whereas the 

 latter was not published until the early part of the following year. In noticing this circumstance elsewhere 

 (vide Annals and Magazine of Nat. Hist., vol. xviii, p. 242), I find I have incorrectly stated, that No. 55 of 

 the 'Mineral Conchology' was published in Dec. 1823. One great proof of the necessity of adopting the 

 genus Axinus, as typified with A. angulatus, is in the fact that two or three species belonging to it have 

 been made typical of three or four synonymous genera ; for example, Axinus, Cryptodon, Ptychina, and 

 (?) Clausina. 



