86 CUETACEOUS PELECYPODA 



(listino-uislied by its marked solid structure and the interrupted pallial line, but 

 especially by the very elongated and curved form of the posterior muscular 

 impression, which in Fanoiicea, as here restricted, is always rounded. The pallial 

 sinus also is in both genera very different; further, Ave have in the hinge of 

 PanopcBa in each valve one tooth which fits into a simple emargination of the 

 inner hinge margin, while in Glycimeris (norveyica) we have a tooth which fits 

 into a special depression of the frontal area of the hinge. Considering these 

 characters, which are quite as well expressed in the tertiary form, as they are in the 

 recent one, I believe there is sufficient reason for a generic distinction. 



8. Cyrtodaria, Daudin, 1799. Shell solid, rather compressed, gaping at both 

 ends, more widely in front, equivalve, inequilateral, the posterior portion being 

 much shorter than the anterior ; pallial line interrupted and irregular ; sinus very 

 small ; hinge-region thickened, but without teeth ; fulcra supporting the ligament 

 strongly prominent. The surface of the shell is covered with a thick epidermis, 

 which has quite the character of that of the solemyid^, and forms a passage to 

 the soLENiDyE. With this last named family,- at least with several typical genera 

 belono-ing to it, as, for instance, Tagelus and others,- Cyrtodaria has in common the 

 shortness of the posterior region of the shell. The resemblance is still more promi- 

 nent when we compare with it old specimens of Tcigelus or NovacuUna, in 

 which the hinge-teeth have become obsolete, which is by no means unusual. The 

 internal structure of the shell with its irregular pallial line and the form of the animal 

 clearly point out the great resemblance of the present genus to the previous one. 



There is, as I have already noticed, a gradual change to be observed in the posi- 

 tion of the beaks in the different genera of this family. In Cyrtodaria they are 

 situated at about one-third of the total length from the posterior end, in Ghjcimeris 

 they are nearly central, in Panopcea they are placed at about two-thirds of the total 

 length from the posterior end, and in Saxicava they are almost anterior. — Only one 

 recent species of Cyrtodaria is known to inhabit the arctic regions. In external 

 form it undoubtedly closely resembles Solemya, and the latter could be treated of in 

 this place were the animals not so thoroughly different from each other. The animal 

 of Solemya is allied to that of Crassatella, and though the two genera are at 

 present as thoroughly distinct as two shells could possibly be, there are a number 

 of very old fossil forms, like Orthonota, Leptodomus, Sedgioickia, and others, which 

 appear to connect both. 



List of cretaceous species. 



For reference to the species known from cretaceous rocks see Pictet and Campiche, Pal. 

 Suisse, 4"^ ser., S"" part., p. 67, &c. 



1. — Saxicava anliqua, d'Orb., is only known by a phrase insufficient for the identification of 

 the species. Dujardin also suggested that a cast of a small species from cretaceous beds of the 

 Touraine may belong to a Saxicava. 



2. — S. Carolina, n. sp., occurs in our South Indian cretaceous deposits. 



PANOP^A. 



Pictet and Campiehe state (loc. cit., p. 46,) that all the species referred to Agassiz's 

 Mppsls, which Terquem has shown to be in part the same as Flearomya, differ from this genus by 



