360 CRETACEOUS PELEOYPODA 



hinge toothless, with the areal margin more or less straight, and a very slight liga- 

 mental furrow ; ventral margin in front of the middle generally insinuated, the 

 posterior part of the shell being often much wider and higher than the anterior; 

 anterior muscular impression distinct and large ; type, Jf. (Cypricardites) modio- 

 laris, Con. 



Hall does not mention the existence of a posterior muscular scar, and thinks 

 that Modiolopsis possesses only one adductor muscle ; but judging from the very 

 great similarity of the shells of Modiolopsis and Modiola, it seems very probable 

 that a posterior adductor also exists in the present genus, but it may be represented 

 by only a very narrow and faint impression along the posterior upper margin of the 

 shell, difficult to be traced in fossil specimens. In many recent Modiolce this pos- 

 terior adductor is also barely traceable, and those shells are, strictly speaking, 

 undistinguishable from Modiolopsis, unless the great size of the anterior adductor 

 is really so prominent and constant a character as it is stated to be, though this 

 appears to be somewhat doubtful. The genus includes only palaeozoic,- mostly 

 Silurian,- species. 



2. Ilippomya, Salter, 18 ? ? (Woodward's Man., 2nd edit., p. 423). "Shell 

 gibbous, with anterior inflated close beaks, a long cardinal edge ; anterior edge 

 short and separated by a strong sinus from the inflated posterior ridge and slope." 

 One Devonian species is said to be known ; I am not acquainted with it. 



3. MyoGoncha, Sow., 1825, (Min. Conch., V, p. 103, pi. 467). Very 

 elongated, anteriorly narrow, with terminal, or sub-terminal, beaks, of solid 

 structure ; hinge sometimes almost toothless, or with an elongated, upper cardinal 

 tooth in each valve, and sometimes with a similar remote posterior lateral ; liga- 

 ment external, placed in a long and deep groove, and supported by strong fulcra ; 

 anterior muscular impression small, situated near the interiorly thickened ante- 

 rior end, often margined posteriorly by a raised rib ; a small impression above 

 and behind the anterior muscular scar indicates the presence of a strong pedal 

 muscle, and this again would seem to indicate a strong foot ; posterior muscular 

 scar much larger than the anterior, roundly oval; pallial line entire; surface 

 concentrically striated, generally with a few radiating ribs along the posterior 

 upper margin; type, M. crassa, Sow., from middle Jurassic rocks. 



The first species of this genus appear in the upper carboniferous and Permian 

 deposits ; these species have the hinge-teeth very slightly developed and sometimes 

 almost obsolete, but the anterior muscular scar is well marked. They strongly 

 resemble Modiolopsis and seem to replace that genus. 



Deshayes (Paris foss., 2nd edit., I, p. 752,) identifies Myoconcha and 

 Sippopodium with Gardita, which seems to me to be inadmissible ; and there is 

 scarcely any student of fossil conchology who will, I think, now adopt Deshayes' 

 views. Myoconcha forms a very good genus by itself, characteristic of the upper 

 palaeozoic and mesozoic deposits ; the few species noticed from the tertiaries are 

 doubtful. Gray's suggestion to classify the genus in the family Dreissenid^ 

 seems more correct. But as Myoconcha has the region upon which the anterior 



