230 CRETACEOUS PELECYPODA 



resembles Monopleura, differing from it by the superposition of the teeth in the 

 upper yalve, but the form of the shell is intermediate between Dicer as and Chama. 

 The structure of the shell is the same as in the former, the reticulated (outer) layer 

 being laminar, the prismatic (one or two) which is somewhat thinner usually 

 changed into crystalline limestone. The species of Eeqidenia are as yet known only 

 from cretaceous strata. 



4. Monopleura, Math., 1842, (ibidem, p. 105). Shell inequivalve ; lower valve 

 larger, sub-spiral, sometimes conical, with the termination attached^ upper valve 

 smaller, more or less depressed, sometimes almost flattened like- an operculum, but 

 always with a marginal sub-spiral beak ; lower valve usually with two internal more 

 or less prominent ribs, one median, the other sab-posterior; hinge of the lower 

 valve consisting of a single sigmoid elongated tooth, and two shorter teeth in the 

 upper or flatter valve ; these teeth are anterior and posterior, not upper and lower as 

 in Requienia ; sometimes there is a small posterior approximate lateral tooth pre- 

 sent in either valve ; muscular impressions two, elongated, pallial line entire ; liga- 

 ment external situated in an umbonal groove in the lower valve and extending to 

 the hinge, where it is attached to the inner side of a slightly projecting hinge 

 margin of the smaller valve. Type, M. Valangiensis, Pict. and Camp. 



In form and sometimes also in ornamentation the MonopleurcB greatly resem- 

 ble BeqiiienicB, but the hinge line is thicker, and the teeth in the upper valve are 

 placed one behind the other, while in Requienia they are one above the other, the 

 result being apparently obtained by the valves being considerably more spirally 

 twisted. Only cretaceous species of Monopleura are as yet known. 



5. Lycodus, Schafhgeutl, 1863, (Siid-Bayerns, Leth. geog., p. 375). Shell in- 

 equivalve, oblong, moderately tumid, with large incurved and rather approached 

 beaks, of solid structure, and concentrically costated on the surface. In the place 

 of the hinge there seems to be in the left valve a large hinge-plate, the anterior 

 portion of which is partially elevated and prolonged into a transverse tooth, and 

 the posterior depressed, probably for the reception of the tooth of the other valve. 

 Prom the posterior part a rib runs internally up to the umbones, and there is also a 

 posterior elongated tooth present almost parallel to the margin of the shell. 



One species, L. cor, is figured and described from an Alpine limestone bed of 

 the Watzmann ; the formation is not stated in detail, but it is probably lower secon- 

 dary. It is difficult to form a correct idea of this fossil, as the hinge is as yet far 

 from sufficiently known, but it seems to indicate a new form of the Chamidm, 

 allied to Chama in external appearance, and to Caprina in the hinge. Single valves 

 have a considerable resemblance to those of JPachyrisma, " * 



6. Chama, Linn., 1758. Shell inequivalve, attached by- one or the other 

 valve, the free valve being, as a rule, the smaller one; surface usually richly 

 ornamented with lamellae and spines ; hinge of the attached valve consisting of 

 a deep curved furrow bounded above by a small ridge and below by a strong tooth, 

 free valve with a single large corrugated sometimes flexuous cardinal tooth ; a small 

 posterior lateral tooth is usually present, situated close to the cardinal ; muscular 



