276 CRETACEOUS PELECYPODA 



besides these there is in the right valve a large rounded anterior cardinal tooth cor- 

 responding to a cavity in the left valve ; anterior muscular impression close to the 

 hinge and somewhat irregular, posterior large, with an internal raised margin. 

 Type, Conchodon infraliasicus, Stopp., from the '^ Avicula contorta -heds,'' The 

 species for which this genus has been proposed greatly resembles that which 

 Giimbel called Fachy-megalodon, but if the hinge, as figured by Stoppani, is cor- 

 rect, there is considerable difference between both. Stoppani says that there 

 exists an internal ligament, lodged along the posterior margin ; the figure of both 

 the valves does not give any indication of such an internal ligament. 



6. Fachyrisma, M. and Lycett, 1850, (Moll, of the great Oolite, pt. ii, 

 p. 78). Shell cordiform, very solid, with prominent incurved beaks, concentrically 

 striated, hinge area very large, with two slightly oblique cardinal teeth in each 

 valve, the posterior being much larger than the anterior, which in the left valve 

 is situated at the lunuJar margin ; muscular impressions moderate, both very 

 close to the hinge, the anterior deep, oblong, with a very shallow pit above it and 

 separated from the large impression by a small raised tooth ; the posterior muscular 

 impression is situated upon a raised plate close to the hinge; ligament strong, 

 supported by fulcra. 



Type, P. grande, M. and L. This appears to be the representant of the 

 Megalodons in Jurassic rocks ; it seems to form a connecting link between them 

 and the next genus. 



7. Opis, Defr., 1825, (Pictet Traite de Pal., tom. iii, p. 305). Shell trigonal 

 or cordate, with very prominent, attenuated, incurved beaks and large excavated 

 lunula ; hinge with one long cardinal tooth in the right and two in the left valve, 

 the anterior tooth forming the internal edge of the lunula ; muscular impressions 

 of moderate size, oval, ligament long, linear. Type, O. hmulata, Sow. This is a 

 characteristic mesozoic genus; no species have as yet been found in tertiary 

 rocks. 



8. Opisoma, Stol., 1870. Shell trigonal, much higher than long, with long 

 attenuated slightly incurved and approached beaks, lunule large and deep ; hinge 

 with three elongated cardinal teeth in each valve, and a small posterior lateral 

 tooth situated above the posterior muscular impression, which is only slightly 

 larger than the anterior one, both being deeply excavated. Type, Opis paradoxa, 

 Buvignier, (Stat. Geol. de la Meuse, p. 17, pi. xiii, figs. 37-42). Species refer- 

 able to this genus occur in upper Jurassic and cretaceous rocks; in form they 

 perfectly resemble some species of Opis, but the hinge is considerably different, 

 possessing many more cardinal teeth than are to be observed in that genus. 



7^ 9. Far-astarte, Conrad, 1862, (Proc. Phil. Acad., p. 288) ; I have not been 



able to procure more than the reference to this genus. 



10. C'jpricardinia, Hall, 1859, (Pal. New York, iii, p. 266). Shell elongately 

 trapezoidal, moderately compressed, beaks anterior or sub-anterior, slightly pro- 

 minent, surface concentrically lamellated or striated, with a posterior more or 

 less distinct dorsal ridge; the ligament appears to have been thin; muscular 



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