JUKES-BEOWNE : CRETACEOUS AND EOCENE TENERID^. 167 



two groups I should agree with Dr. Dall in ranking Cyclorisma as 

 merely a subgenus (or oven a section) of Cyprimeria. 



It appears, however, that Cyclorisma varies considerably in shape, 

 the typical form, as well as many of the Indian species, being nearly 

 orbicular ; others, like the British Cretaceous shell which has hitherto 

 been known as Ve^ius vedensis, Forbes, are suborbicular, and lastly 

 we have the allied species Venus f aha, Sow., which has the essential 

 hinge-characters of Cyclorisma, but resembles a Ta^jes in shape. 



I cannot find that any adequate description of the hinge of 

 Cyclorisma has yet been published. The following has been drawn up 

 partly from the figures of Stoliczka, but mainly from an examination 

 of specimens of C. xectensis preserved in the Sedgwick Museum at 

 Cambridge. There are three cardinal teeth in each valve, the posterior 

 of the right being always deeply bifid, and the left median being some- 

 times bifid. There is no anterior lateral, but the hinge-plate is 

 I)rolonged anteriorly, and forms a more or less concave space in front 

 of the anterior cardinal very like that in the hinge of dementia. 



In the right valve the two anterior teeth are nearly parallel, and 

 are directed forward, so that sometimes the anterior seems to end in 

 the concavity, Avhile the median runs into the rim of the hinge-plate. 

 The posterior tooth is not merely bifid, but is actually double, 

 consisting of two separate laminae, one of which curves forward to 

 meet the top of the anterior tooth (as in Pitaria), while the hinder 

 one runs in a nearly straight but oblique line across the hinge-plate. 

 In some species this division of the posterior cardinal is so complete 

 that the hinge seems to consist of four separate teeth ; even Stoliczka 

 w^as thus deceived, for he describes the hinge of the right valve as 

 consisting of "two diverging pairs of thin laminar cardinal teeth, 

 each pair originating from one point, and thus preserving the 

 character of a single bifid tooth." ' In the left valve the anterior 

 and median teeth are united at the top and form a A - shaped tooth 

 Avhich barely reaches the margin of the shell ; the posterior tooth is 

 entire, not very long, and is slightly curved so as to reach clown to the 

 inner margin of the hinge-plate (as in J)osi?iia and some species of 

 Pit aria). 



The hinge of C. faba only difi'ers from the above in regard to the 

 relative strength of the left anterior and median teeth. In vectensis 

 the median is the thicker, and may have been grooved ; in faha both 

 teeth are entire and equally strong, 



No true Cyclorisma seems to have survived the Cretaceous period, 

 at any rate in Western Europe, but some of the Eocene shells, now 

 known b)- the name of Afercimonia, resemble Cyclorisma in the bifid 

 structure of the posterior cardinal and in the anterior concavity of 

 the hinge-plate. 



14. Clementia, Gray, and Fl.wentia, n.subgen. PI. VI, Fig. 10. 



A species of Clementia exists in the higher part of the Parisian 

 Eocene series, and was described by M. Cossmann in 1886 (op. cit., 



' " Cret. F auna of South India " : Pal. Indica, 1871, vol. iii, p. 157. 



