208 CRETACEOUS PELECYPODA 



beiusr svamilate. Such can be seen on both ends of the shell of C. orhita for 

 instance, which is a Trachycardium ; I don't see, therefore, the necessity for a new 

 sub-genus. 



If/. Cerastoderma, Morch (ex-Poli,) 1851. Shell broadly ovate or longitudi- 

 nally cordate, inflated, inequilateral, posterior side somewhat longer and nearly closed 

 at the end ; surface ornamented with numerous closely set thick ribs ; cardinal teeth 

 small, the upper ones sometimes imperfectly developed ; laterals distinct. Type 

 C. ediile, L. Species of the same type seem first to be met with in the Em-opean 

 cretaceous rocks ; they are rare in the Eui'opean eocene, but more common in those 

 of Asia; of recent species Romer quotes only three, identifying with C. edule 

 several forms which formerly were considered as distinct. 



2. Fapyridea, Swainson, 1840. Shell elongately ovate, posterior side sen- 

 sibly longer than the anterior, more or less gaping ; of a rather thin structure ; hinge 

 line long, straight, or slightly curved ; surface radiately ribbed, ribs more or less 

 granulated or spinose, pallial impression mostly sinuated ; upper cardinal tooth of 

 left valve sometimes obsolete, or nearly so, (Card, hullatum, Linn.). This type is 

 hardly represented in cretaceous rocks, a few species are known from the tertiaries, 

 and of recent ones only four are referred to it by Homer, — Cm'd. btdlatum, rngutum, 

 ringictdum, and papyraceum, (loc. cit., p. 74, &c.), but it is not evident why such 

 apparently distinct forms as C. btdlatum and Ciiniingii should be referable to one 

 and the same species. The thin structure of the shell which is posteriorly 

 produced and the usual presence of a pallial sinus distinguish this genus from 

 Cardium. 



I have again to mention Meek's Liopistha (see p. 71 and Smith. Misc. Coll., 

 No. 177, 1864, pp. 12 and 32). Meek says, — "the shells embraced in this group 

 Avhich seems to be peculiar to the cretaceous system have, according to Mr. Conrad, 

 the hinge of Papyridea, Swains. They differ, however, from the type of that genus 

 fC. hidlatum, Linn.,) in being closed and without costte on the postero-dorsal 

 region, or crenulations in the posterior margins of the valves. They are also much 

 thinner shells." Romer's Card, elegantulum from Texas is considered as the 

 type of this so-called new genus, and Meek in his list (loc. cit.) adds three others. 

 It is absolutely necessary to know upon which species Mr. Conrad made the observ- 

 ation regarding the similarity of the hinge with Papyridea, or his observation must 

 be at least confirmed by the examination of some closely allied species. I can 

 at the present only say that the well known Pholadomya caudata, Romer, 

 or Ph. (equivalijis, Goldf., which has (see p. 79) repeatedly been described as 

 a Cardium has not the hinge of any of the Cardiidm, but that of Pholadomya. 

 I have seen specimens of it which hardly gape at all posteriorly, so that this 

 character can be of no great value in the Texas Curd, elegantulum of Romer. 

 For my own part I cannot suppress my great doubts as to the propriety of 

 the so-called Liopistha, though looking at the various forms of Papyridea and 

 others described under Didacna, Monodacna, &c., there is nothing impossible in 

 Conrad's statement. 



