186 PERMIAN FOSSILS. 
shells—the Aainus angulatus of the London Clay, and the 4. obscurus of the Magnesian 
Limestone. Both species agree remarkably well in their marginal outline—one side or 
extremity bemg 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. Avinus angulatus 
has the cartilage placed on the rounded side of the umbone; whereas in 4. odscurus 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), 4xinus angulatus possesses the characters of certain 
Lucinide ; whereas A. obscurus agrees with Zrigonia. Now, as Mr. J. Sowerby dis- 
tinctly stated, that-he considered the London Clay Avinus angulatus to be “the type 
of the genus,”! it follows that the so-called Avinus obscurus must be removed to some 
other generic group. It is herein placed in the present genus. 
Schizodus i1 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 AZyophoria Goldfussi. The thick posterior tooth of this species may 
be supposed to be the homologue of the bifid tooth of the corresponding valve of 
Schizodus 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 MJyophoria, 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 Zrigonia as largely developed examples of their kind, and in the 
light in which they are described by Agassiz, not Lamarck and others, without attending 
1 Mr. Morris appears to have been the first to suggest that Avinus angulatus belonged to Turton’s genus 
Cryptodon (vide Catalogue of British Fossils, p. 80); probably on account of its close resemblance to 
Cryptodon flexuosus, Montague, which is by some considered to be a species of Lucina. Considering the 
type of Lucina (viz. Venus Jamaicensis, Chem.), Montague’s species evidently belongs to a distinct genus: 
if this be admitted, the name Aainus 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. xvii, p. 242), [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 Avinus, as typified with 4. angulatus, is in the fact that two or three species belonging to it have 
been made tynical of three or four synonymous genera; for example, Awinus, Cryptodon, Ptychina, and 
(2) Clausina. 
