330 CEETACEOUS PELECYPODA 



more prominent along the median line where the two valves meet, and the striation 

 is as compared with the size of the shell coarser. The shell consists of two distinct 

 and easily separable layers ; the outer, thick, calcareous, which exhibits the above 

 noted ornamentation, and the inner, thin, pearly layer, which is radiately striated 

 externally and smooth internally, only the inner peripherical margin being denticu- 

 lated. When the upper calcareous layer of the shell is removed and the pearly one 

 preserved, it would be hardly possible to identify both (see figs. 24 and 25),* if 

 their connection was not verified by other specimens which have portions of the 

 upper layer still preserved. Again, when both the calcareous and pearly layers are 

 removed, the cast appears as a quite smooth shell. 



This is a sufficient exemplification of the value to be attached to species which 

 are described only from imperfect casts. They are sensu stricto useless. 



Locality .—KomYidiioov, in dark earthy limestone. 



Formation, — Ootatoor group. 



XXXVI. Family,— AB CIDJE. 



The mantle is separated in its entire extent, generally with fringed edo'es 

 sometimes provided with small ocelli, which appear to represent eyes. As a rule, the 

 edge of the mantle is double ; the outer is thin, entire, and secretes the epidermis • 

 the inner is thicker, crenated, or undulated, and if any occelli are present they are' 

 placed on the outer side of this inner edge. The foot is large, geniculate, more or 

 less extensile into a disc and anteriorly generally somewhat produced, below often 

 grooved, posteriorly truncate with or without a byssus ; the palps are said to be 

 short and represented by merely detached portions of the branchia, which are two 

 in number on either side, consisting of numerous rather loose filaments ; they are 

 generally very large and sub-equal ; there are no special siphons present. 



The shells are round or oval and elongated, with an external ligament usually 

 attached to a special flattened and grooved area below the beaks, sometimes con- 

 centrated in a single pit ; the hinge is composed of numerous cross sub-equal teeth, 

 fitting alternately beside each other; two large muscular impressions, pallial sinus 

 entire. 



There are two somewhat difi'erent groups or sub-families to be distinguished, 

 AxiN^iN^ C=pbctunculinmJ and arcinje. The former more resemble the pre- 

 vious families than do the latter. The animals of the axin^in^ have a short foot 

 with a broad disc and without a byssus. The shells have the hinge-teeth always 

 arranged in a curved unbroken series, equal on both sides, but as regards the form 

 of the shell and the development of the ligament they partially greatly resemble 

 NucuLiB^, The genera referable to the sub-family are mwunella, Trigonocoelia, 

 Limopsis, Cyrilla, Nucinella, Axincea, and Lyrodesma. Of these the fossil Trigo- 

 noGCBlia strongly recalls the shape of some of the NjicTiLAmiyM ; its ligament is 

 sub-external, and Gyrilla appears to be its recent representative; Nucinella has 



* This valve has somewhat snfFered by accidental pressure, and appears, therefore, flatter and more produced above 

 than the shell would be in its perfect state. ' 



