BILLINGSELLID^. 767 



select a series illustrating the great range of variation in form and surface markings, first, from 

 Sweden; second, from Newfoundland; and third, from New Brunswick. I was not able to 

 obtain satisfactory material from Wales, but Davidson's figures [1869, PI. XXXIII, figs. 22-28] 

 illustrate fully the peculiarities of the species as it occurs there. 



The general form of the shell is fairly well shown by the series of figures on Plate XCVlII ; 

 also the great variation in surface characters. Only one specimen was found in which the two 

 valves were united (PI. XCVIII, figs. 6, 6a-c). The convexity of the two valves is subequal, 

 the ventral being a trifle greater. The cardinal view (PI. XCVIII, fig. 6c) shows how short the 

 hinge line of both valves is in a specimen where the cardinal extremities are rounded. The 

 side view shows the angle of slope of the areas of the two valves. 



A very careful search of the casts of the interior was made for the purpose of illustrating 

 the muscle scars and vascular markings, and in a few instances traces of these have been found. 

 In one illustration (PI. XCVIII, fig. le) the area (pseudospondylium) beneath the umbo of 

 the ventral valve is outlined, also the main vascular sinuses; in another (PI. XCVIII, fig. Ip), 

 drawn from the cast of a small ventral valve, the area, main vascular sinuses, and the details 

 of the umbonal cavity are clearly shown. The backward extension of the cast of the grooves 

 occupied by the sinuses to the apex of the cavity is ver};- distinct; also the area between them 

 occupied by the path of advance of the scars of the adductor muscles. Matthew [1892, PI. 

 XII] illustrates some traces of the vascular markings and muscle scars, but they are almost too 

 indefinite to be of value. 



In a single specimen of the ventral valve from Oeland Island (PI. XCVIII, fig. Ip) the cast 

 of the umbonal cavitj^ is nearly as sharp as in some specimens of Eoorthis remnicha. The 

 cavity is tripartite (PL XCVIII, fig. Ic), the central division being occupied by the diductor muscle 

 scars, and the two lateral divisions are continuous with the ridges representing the casts of the 

 main vascular sinuses. I have been unable to secure any illustrations to show clearly whether 

 there is a deltidium present or not. The delthyrium is clearly shown in several specimens. 

 Casts of the interior of the dorsal valve from Newfoundland (PI. XCVIII, figs. 2f and 2h) show 

 the presence of crural plates that extend to the bottom of the valve; also short but very 

 definite crura and small depressions beside the latter for the reception of the teeth of the ventral 

 valve. A cast of the ventral valve (PI. XCVIII, fig. 2d) shows that the dental plates extend 

 to the bottom of the valve; and specimens from Sweden (PI. XCVIII, figs. Ic and le) show 

 that in certain individual ventral valves the dental plates and a transverse arching ridge sharply 

 defiiie the umbonal cavitj^. 



The average size of the specimens from Oeland Island is about 5 by 6 mm. One dorsal 

 valve is 7 by 9 mm. Those from Hunneberg average from 5 to 6 mm., with occasional shells 

 7 by 8 mm. In Newfoundland great numbers measure 4 by 5 mm., and some have a length 

 of 10 mm. and a width of 12 mm. Matthew [1892, p. 48] states that the average size of the 

 shells in the Acadian rocks is about 5 by 6 mm. Brogger [1882, p. 48] mentions an example 

 from Toien, Christiania, Norway, measuring 10.5 mm. by 12.5 mm. 



I have not attempted any detailed description of the species, as it is so variable in form 

 and surface markings. An attempt was made to determine varieties in the material from 

 Sweden and Newfoundland, but there was such a gradation in all of the characters upon which 

 such differentiation might rest that it was finally abandoned. Matthew [1892, p. 48] considers 

 that he has reason for establishing varieties among the New Brunswick forms. Whether these 

 would be of value if a large amount of material were available for comparison is doubtful. 



Kaj^ser [1876, PL I, figs. 11 and 12] illustrates a shell that is apparently identical with 

 Onisia lenticularis, which occurs in the sandstone at Tilcuya, Province of Jujuy, Argentina, 

 in association with Olenus, Agnostus, etc. He also illustrates [1876, PL I, fig. 13] a dorsal valve 

 of a small Orthis that in many respects looks more like 0. lenticularis than figures 11 and 12 of 

 the same work. I very much doubt if they represent this species; in fact, Kayser questions 

 his identification by an interrogation mark. He says of the shell [1876, p. 9]: 



Another, much smaller, Orthis species occurs in the fine-grained sandstones of Tilcuya, associated with Olenus, 

 Agnostus, Arionellus, Theca, and Orthis saltensis, which has just been described. The smaller Orthis species is 



