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mantles inargius both families must have been very simihtrlj^ oi'g'aiiised. AVoodwarcl 

 says, that Eippurites possesses a distinct pallial sinus. It is nothing extraordinary 

 that any one should see in tlie posterior internal folds or flexures the indication of a 

 pallial sinus, and, strictly speaking, there is nothing very much opposed to tliat 

 explanation, exce23t that we know nothing exactly similar to this to occur in other 

 Pelecypoda. The shells consist of two very unequal and dissimilar valves, of which 

 the attached one is the larger, more or less conical and sub-spiral ; the upper is oper- 

 culum-like, flat or slightly elevated with a central or sub-central apex and concen- 

 tric striae of growth ; the hinge-teeth are variable, but apparently always more 

 developed and more complicated in the free valve ; there is no distinct trace of a 

 ligament. The outer reticulated layer of the shell is mostly developed in all Uip- 

 jxuritidce ; the inner layer is usually thin, sometimes hardly traceable. 



All the species as yet known are from cretaceous deposits, with the exception 

 of the peculiar Tamiosoma which is from the tertiary Californian deposits, and is 

 as yet only doubtfully referred to the present family, (vide postea). 



The cretaceous species have l)een sub-divided into the three following genera. 



1. RadioUtes, Lam., 1801, (vide Bayle in Bull. Soc. Geol., France, 2nd 

 ser., xii, p. 793). Lower valve conical, attached near the umbo, very solid. The 

 surface usually strongly lamellar, margins ribbed and widely expanded, internally 

 often with two grooved longitudinal ridges into which the edges of a curved 

 widely projecting cartilage-plate fit, but these grooved ridges themselves do not 

 usually extend to the margin of the valve. The cartilage-plate is inserted near the 

 centre of the free valve and extends far below and obliquely towards the base of 

 the attached valve without, however, touching it except at the lateral edges which 

 are often serrated. The size and extent of this cartilage-plate vary greatly in 

 different specimens, and the ridges in the lower valve seem to become sometimes 

 obsolete. The cartilage itself is apparently situated between this projecting, inter- 

 nally concave, striated plate and the attached valve ; the muscular impressions are 

 situated next to the cartilage-plate ; in the lower they are large, but not particularly 

 marked ; in the upper smaller valve they are situated on solid prominences, these 

 being more or less roughly striated; pallial line entire. 



A peculiarity of the lower valve of RadioUtes is its usually strongly lamellar 

 surface, the thickness of the shell being penetrated by numerous branching canals ; 

 some specimens are on the surface also longitudinally ribbed, but this ornamentation 

 more often occurs in SpJuendites. Another peculiarity of the same valve is the 

 large longitudinal area which extends from the beak to the ventral side and is 

 usually destitute of the laiiicllae and ribs which occur on the rest of the surface. 

 This area does not correspond to the hinge area of Ostrea or Spondyliis, as it would 

 appear to do at first sight, for the hinge is situated to the left, or inferior side of 

 it ; what organ or particular edge of the mantle has produced it I am unable to 

 conjectiu*e at present. 



The smaller valve is rarely equally richly ornamented as the larger ; it is most 

 conunonly smooth with fine concentric striie of growth. 



