or SOUTHERN IKDIA. 427 



Fossil species first appear in the lowest of cretaceous deposits, and some of 

 them have a very great, almost general, geographical distrihution in the higher 

 strata of this formation. Dm-ing the upper tertiary and the recent ej)och, the 

 species attain a larger size than any other Pectinid^. 



6. Hemipecten, Adams and E.eeve, 184-9, (Proc. Zool. Soc, p. 133, Reeve, 

 Iconica, vol. VI). Shell somewhat irregularly orhicular, inequivalve, valves very 

 thin, hyaline ; posterior ears not well developed, anterior better defined, flattened, 

 that of the right valve with a very deep hyssal sinus, which has the lower edge 

 denticulate ; hinge-line almost straight, vrith a thin marginal ligament, internally 

 edentulous, and with a small subcentral marginal cartilage pit ; right (or usually 

 called lower) valve flatter than the left one, which is more or less convex. Type, 

 Sem. Foi'besianus, Ad. and E-., from the Sooloo Archipelago, is the only recent 

 species known. It appears to adhere temporarily by the right flattened valve to 

 sub-marine objects ; its thin hyaline stru.cture is very characteristic, and so is 

 also the small marginal cartilage pit. 



I believe that several of the fossil mesozoic species described under the name 

 Hinnites, but distinguished from typical forms of this genus by a greater inequality 

 of the valves and thin structure, are referable to Semipecfen, and the same may 

 be the case with some of the Jurassic Fl((cuiiop)sis ; but the materials upon which 

 particularly the latter genus has been founded are in many respects so insuffi- 

 cient that it is extremely difficult to institute a proper comparison of these shells 

 with Anomia. 



7. Hinnites, Defr., 1821. Valves more or less solid, radiately ribbed exter- 

 nally and often foliated, right valve partially attached, often slightly larger and 

 irregular ; area between the hinge and the beaks more or less developed, striated, 

 traversed by a central cartilage groove, somewhat extending internally ; hinge eden- 

 tulous ; hinge-line unequally eared on both sides, the right valve with a byssal sinus, 

 Si's, in Pecten ; muscular scar sub-central, large ; type, ^. Cortesii, Def., a tertiary 

 fossil species. This genus is represented from the oldest sedimentary deposits through 

 all the periods, but only about four recent species are known ; it appears to have had 

 its greatest development during the mesozoic epoch. 



The character of the shell strongly recalls that of Pecten, differing from it 

 by the sessile habitat at certain stages of age, and by the peculiar development of 

 the hinge area in the left valve. Prom Terquemia it differs by the lateral ears and 

 the byssal sinus. It appears to have preceded Spondylns in geological history, and 

 forms an intermediate link between this genus and Pecten or Ostrea, while Ter- 

 quemia connects in a similar manner Plicatula and Ostrea. The thin, almost 

 hyaline, fossil species of Hinnites may, as already noticed, partially belong to Hemi- 

 pecten. 



List of cretaceous species of pectinid^. 



Pictet and Campiehe (Pal. Suisse, V""" ser., 4™^ part., p. 177 et seq.), class the species of this 

 family under three generic names : Pectoi, Janira (^= Vola), and lUnnlles. lu the subsequent list 

 I shall note the sub-generic names in parentheses^ and make a few additions. 



