168 PROCEEDINGS OF THE MALACOLOGICAL SOCIETY. 



p. 115) under the name of C. Deshayesi. It agrees in all essential 

 characters with the recent typical species (C. papyracea, Gray), and 

 has a similar thin fragile shell. 



Nothing like a true Cleynentia has yet been found in beds of 

 Cretaceous age, but the species which has hitherto been known by 

 the name of Venus ovalis, Sow., offers many points of resemblance. 

 In shape and general aspect V. ovalis resembles a CaUista ; indeed, 

 the resemblance to Callistina plana is so great, not only in shape, 

 but in the arrangement of the cardinal teeth, that unless the anterior 

 part of the hinge-plate was exposed, the shell might easily be mistaken 

 for a small specimen of C. plana. The place of the anterior lateral 

 and its corresponding pit is taken by a concave space of elongate 

 triangular form, similar to that on the hinge-plate of dementia. The 

 pallial sinus is deep, ascending, and rounded. 



The only other genus with which it can be compared is Cyclorisma, 

 but the position of the teeth is quite sufficient to distinguish them, 

 for in C. ovalis the anterior and median cardinals cross the hinge- 

 plate nearly vertically, instead of being directed obliquely forward as 

 in Cyclorisma, while the posterior, though bifid, is not the double 

 tooth of Cyclorisma. 



From dementia it differs, not only in the greater thickness of the 

 shell, and consequently of the hinge-plate, but also in the form of 

 the right posterior cardinal, which in dementia is a short, straight, 

 narrowly bifid tooth, consisting of two equal united laminae ; while 

 in V. ovalis this tooth consists of two diverging laminae, the hinder 

 of which is much longer than the other; it is, in fact, exactly 

 like the posterior tooth of Callistina plana. Lastly, V. ovalis has 

 a circumscribed lunule. 



In other respects the shell agrees with dementia, and I am 

 certainly of opinion that it may be regarded as the ancestor of this 

 genus, but the differences are such that I think it should rank as 

 a subgenus, and consequently should have a distinctive group-name. 

 As the best specimens come from the Blackdown Beds, and are often 

 of a golden-yellow colour, the name may be taken as indicative of 

 that colour, and also as being formed on the same model as dementia. 



I think it very probable that some other Cretaceous species will 

 have to be associated with F. ovalis, such as the Tapes faba of 

 Holzapfel {non Sow.), from the Gosau Beds, as well as some French 

 Lower Cretaceous species, but these latter cannot yet be determined, 

 because their hinges are at present unknown. Again, a recent shell 

 from the west coast of America has been referred to dementia by 

 Dr. Dall,^ under the name of CI. solida, the single right valve known 

 being described as "large and solid for the genus" ; but the teeth of 

 this unique valve are neither like those of other dementia nor those 

 of Flaventia, so that, till other specimens have been found, its precise 

 affinities can hardly be discussed. 



Finally, there is the remarkable shell which was named Psathura by 

 Deshayes, and of which only two valves are yet known. This only 



1 Proc. U.S. Nat. Mus., vol. xxvi (1902), p. 401, pi. xiv, fig. 4. 



