282 AVICULID^. 



a depression in the other valve ; lunular area long, slightly 

 gaping, with simple thickened margins, posterior side shorter, 

 evenly rounded; muscular scar small, subcentral, semilunar, 

 deeply impressed ; surface of shell 3oncentrically lamellar. 



Chalmasia, Stoliczka, ISTO. 



Diatr. — Several sp. Cretaceous; Europe. G. Tiironensis,Du}. 



Irregularly oblong, longer than high, with prominent obtuse 

 beaks, subequivalve, the valves being slightly convex, ligamental 

 groove large, moderatel}^ excavated, margin in front of the beaks 

 with several irregular incisions, or internal grooves, similar to 

 those of Eligmus, muscular scar subcentral, elongated, and 

 strongly thickened. 



Munier-Chalmas refers the type species to Yulsella, but in no 

 recent or tertiary species of that genus do any incisions or pli- 

 cations occur in front of the beaks, nor is the muscular scar 

 equally strong in any of them. The shell differs from Eligmus, 

 merely by its more compressed form and more centrally placed 

 muscular scar and by its thicker shell ; it shows greater affinity 

 to Pedum, which has, however, only one incision before the beak, 

 and the ligamental pit strongly produced internally. 



Eligmus, E. Deslongchamps, 1856. 



Etym. — Xiy/iij, a sinuosity, in allusion to the sinuosities of the 

 borders of the post-apical opening. 



Distr. — 3 sp. Inferior Oolite, and Great Oolite; Maine-et- 

 Loire, Calvados, Galicia. E. polytypus, Desl. (cxxxii, 85-87). 



Shell free, or perhaps attached by a byssus, nearl}^ equivalve, 

 inequilateral ; ovate or cylindrical, more or less compressed ; 

 anterior extremity' inflated, and shorter than the attenuated pos- 

 terior one. Test rather thick, foliaceous. TJmbones inflated, 

 slightlj^ depressed or flattened, diverging and directed backwards. 

 Yalves closed at both extremities, with an uusymmetrical 

 (byssal ?) sinus behind the umbones ; ornamented by oblique, 

 radiating carinated ribs ; hinge short, straight, edentulous ; 

 ligamental area triangular, with a superficial pit ; muscular scar 

 single, situated on the free end of a spoon-shaped process, 

 which originates from beneath the umbonal cavity ; pallial line 

 wanting. 



The internal process of Eligmus has no analogy with that of 

 theM3^8e and Anatinse, which in them supports the cartilage, and 

 is an internal prolongation of the hinge ; whilst that of Eligmus 

 gives attachment to the adductor muscle, and arises from beneath 

 the hinge. Eligmus is related through Ghahnasia Tiwonensis, 

 Dujardin, to Vulsella; the test, however, is not fibrous, and M. 

 Munier supposes that the internal nacreous layer has been 

 destroyed b}^ fossilization. 



