66 CRETACEOUS PELECYPODA 



19. Mytilimeria, Conrad, 1837. Shell inflated, sub-oval, equivalve, beaks 

 entire, the anterior one cui-ved in, hinge edentulous, with an internal posterior car- 

 tilage groove protected by a small ossicle ; two small muscular impressions ; pallial 

 sinus broad and very shallow. Only a few recent species are as yet known, and 

 from external shape the fossil ones may very easily be mistaken for Modiolcs, or 

 even for species of Lorlpes and allied genera. The fossil EdmondicB and Ceromya; 

 appear to be closely related to the present genus. 



20. mimondia,'Kon\n.c\i, 1842, (Anim. foss. Carb. Belg., p. 66, and King's 

 Perm. foss. of England, p. 162, etc., pi. xx, figs. 1-4). Shell equivalve, transver- 

 sally oval, tumid, slightly gaping in front of the beaks, hinge below the beaks 

 with a raised rib or process, extending posteriorly to near the margin and probably 

 supporting a cartilage; area generally distinct with slightly thickened fulcra for 

 the support of an external ligament. A few palaeozoic species of this genus are 

 only known. The form of the shell also very much resembles that of some species 

 of Lorlpes, with a thin, finely striated shell. 



21. Cardiomorpha, Koninck, 1842, (Foss. Carb. de Belg., p. 101). Shell 

 oval, tumid, hinge edentulous, upper marginal edge behind the beaks somewhat 

 elevated, probably for the support of an external ligament, pallial line simple. 

 This is again a form which, if its characters prove to be correct, can scarcely be 

 distinguished from some Loripes, except by its small muscular impressions and 

 thin shell. I suspect Mc' Coy's genus Leptodomus (Carb. fossils of Ireland, 1844, 

 p. 66,) is identical with it, at least the characters given (loc. cit.) do not exhibit any 

 essential differences. The thin structure of the shell is particularly alluded to. 

 The shell is, however, generically distinct from what Mc'Coy described in 1855 

 as Leptodomus in his work on the palieozoic rocks and fossils. Again, Sandberger, 

 Koninck, and others have described some species as Cardiomorpjha, which would 

 be more properly referable to Mc'Coy's Leptodomus of 1855, probably the same as 

 Loriol's Flectoviya (see p. 69). 



22. Ceromya, Agassiz, 1842, (Etud. Crit., p. 25). Shell tumid, with con- 

 centric strise or ribs, often divaricating at the upper posterior margin ; right valve 

 o-enerally somewhat larger than the left, hinge edentulous ; a shallow groove runs 

 obliquely from the beaks posteriorly and forms behind them a somewhat raised and 

 thickened margin, near which the cartilage must have been situated internally, 

 thou"-li very likely there was also a thin external ligament present ; in the right 

 valve there is besides a small thickening with an adjoining indentation below the 

 beak ; the muscular impressions are very small, the pallial sinus deep. 



The CeromycB are principally characterized by their tumid, thin, and concentri- 

 cally laminated shell, distant beaks, and the oblique furrows which are externally 

 traceable. Chenu in his Manual unites the genus with Koninck's Cardiomorpha 

 under the latter name, but in this I have never observed any such arrangement for the 

 attachment of an internal ligament (similar to that of Lyonsia,) there is along the 

 raised upper margin no trace of a special furrow, and the beaks are closely approxi- 

 mate to each other. Eor the Jurassic beds, especially for the middle series, or 



