330 CRETACEOUS PELECYPODA 



more prominent aloni? 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 sejiarahle layers ; the outer, thick, calcareous, which exhibits the above 

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

 tixternally 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, w^hen both the calcareous and pearly layers are 

 removed, the cast appears as a quite smooth shell. 



This is a suflS.cient exemj)liflcation of the value to be attached to species which 

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



Localitii. — Moraviatoor, in dark earthy limestone. 



Formation. — Ootatoor group. 



XXXVI. FamU!j,—ABCID^. 



The mantle is separated in its entire extent, generally with fringed edges, 

 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 ej)idermis • 

 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 wdth 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 different groups or sub-families to be distinguished, 

 AXiNJEiN^ (= PECTUNCULIN^J and ARciNjE. The former more resemble the pre- 

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

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

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

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

 NucuLiD^, The genera referable to the sub-family are Nucunella, TvigonocoeUa, 

 Limopsis, Cyrilla, Nuchiella, Axmcea, and Lyrodcsma. Of these the fossil Tr'ujo- 

 i/orop.fln strongly recalls the shape of some of the Nuculasidm ; its ligament is 

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



* This v.iIto has soniewliat suflered by accitlental pressure, .iiul appears, therelbro, flatter aud more produced above, 

 than the shell would hu in its perl'ect slate. 



