INVERTEBRATE PALAEONTOLOGY. 7<) 



crowded anteriorly, and larger, more oblique, and broken up into 

 granules behind; surface strongly radiately costate. — Area pfopatula, 



Conrad. 



At the same time that I retain here provisionally, these seven sections as 

 subgenera of Barbatia, I confess that some of them seem scarcely distinct 

 enough to be regarded as subgenera, while it is barely possible that one or 

 two of them (Grfanoarca, for instance) may be properly generically distinct 

 from Barbatia* One difficulty in settling this question with regard to the 

 extinct types, is, that we have no means ot determining whether or not they 

 possessed the peculiar characteristic hairy epidermis of the existing species 

 of this genus. 



Among the more marked differences between Barbatia and Area, to which 

 latter the former is in some respects most nearly allied, may be mentioned 

 the hairy epidermis of Barbatia, with its more arched, shorter hinge widening 

 toward the lateral extremities, where its denticles are longer and more oblique 

 than in Area. The cardinal area is also nearly always much narrower, and 

 the lateral extremities of the valves generally more rounded in outline, than in 

 the latter genus. 



In the present state of our knowledge of the hinge, area, and internal 

 characters of many fossil-forms simulating this genus in external characters, 

 it is scarcely possible to determine, beyond doubt, the geological range of 

 Barbatia. Dr. Stoliczka thinks that it, as well as Area, existed as far back 

 as the Silurian epoch. To me, however, this seems to be an error. At least" 

 all the species with which I am acquainted, from the Silurian and Devonian, 

 resembling these genera, seem to belong to Tellinomya and other groups. 

 In the Carboniferous and Permian rocks, we meet with forms very similar to 

 Barbatia externally; but, so far as known to the writer, they all differ in the 

 characters of the hinge. 



In the Trias, however, we meet with species ayreeinj; pretty well with 

 sections of this genus, among which forms may be mentioned Cucullcta Mur- 

 chisoni of Capellini. In the Jurassic rocks, the genus is more abundantly 



* Cucullwarea, Conrad, is another type even more like Barbatia than his Granoarca in form and gen- 

 eral external appearance*, while its hinge-characters agree exactly with those of Granoarca, excepting that 

 tbe anterior denticles, instead of being very small, erect, and crowded, are as large as those at the other 

 end of the hinge ; and also present the marked difference "t being ranged horizontally, parallel to the 

 hinge-margin, instead of merely very obliquely. According to Dr. Gray, however (Ann. and Mag. N. H, 

 XIX, 2d ser., 319), the degree of obliquity of the lateral denticles in Barbatia is not always of great impor- 

 tance. For instance, he says that in B.glacialis " they are sometimes transverse, like the inner ones, while 

 in other specimens they are nearly longitudinal, like the teeth id' CucullceaJ' 



