OBOLID^. 547 



Dakota are relatively strong and thick. The shell is built up of a thin outer layer and several 

 inner layers or lamellas that, on the anterior and lateral portions of the shell, are oblique to 

 the outer layer and form a thick, laminated shell very much like that of Oholus. Casts of the 

 interior of the ventral valve show a clearly defined area that extends a considerable distance 

 forward along the cardinal slopes. It is divided midway by a strong, rather broad pedicle 

 furrow, and about midway of the very narrow side spaces, by extremely narrow flexure lines. 

 One specimen from the Black Hills shows that the area formed a thin shelf between the pedicle 

 groove and the lateral margins, the undercut extending some little distance beneath it. The 

 area of the dorsal valve is clearly marked in a few specimens; it is relatively short although 

 fairly well extended on the lateral margins. 



The cast of the visceral cavity (v) is rarely preserved except in its posterior portion; this 

 is marked by the cast of a median groove, and the path of advance of the central, anterior 

 lateral, and middle lateral muscle scars. Traces of the muscle scars or the anterior portion 

 of the visceral cavity are rarely preserved; thus the heart-shaped pit so often seen in Lingu- 

 lella and Oiolus is outlined in not over a half dozen specimens out of several hundred casts of 

 the interior. The position and shape of the visceral area varies with the width of the shell. 

 In the long shells it extends far forward, while in the broad shells it scarcely reaches to the 

 center (PI. XLII, figs, le and Ig.) No traces of a median septum have been observed in the 

 ventral valve, but in the dorsal it is shown in the casts as a narrow, clearly defined, sharp 

 depression, extending from between the anterior lateral scars back and between the central 

 muscle scars. 



The muscle scars are more or less clearly shown in a number of casts of both valves. The 

 umbonal scar of the ventral valve is divided as in Oholus, the pedicle scar (m) (PI. XLII, fig. Ig) 

 being situated between the two parts (gg). In the dorsal valve the umbonal scar is situated 

 just in front of the area, arching slightly toward the beak. The scars of the central, anterior 

 lateral, and middle lateral muscles are not clearly defined in the ventral valve, owing to their 

 being crowded together in the space on each side of the anterior portion of the visceral cavity. 



The central muscle scars of the dorsal valve are of medium size and well defined on a number 

 of casts of the interior of the shell. They are situated a little back of the center of the shell 

 on each side of the median ridge, and their longer axis is slightly inclined outward. The anterior 

 laterals are not well defined in the ventral valve, because they are crowded into a very narrow 

 space. On the dorsal valve they are rarely visible, owing to their very faint impression on 

 the shell. The transmedian scars are distinctly shown on the dorsal valve, but in the ventral 

 valve they have not been differentiated from the anterior laterals. 



The markings left on the shell by the vascular system are limited to the main or trunk 

 sinuses on a few casts of the interior (PL XLI, fig. la, and PL XLII, fig. le). 



Observations. — The type specimens described by Conrad [1839, p. 64] were small, and from 

 an uncertain locality in the "Calciferous sandrock." His associate, ]\Ir. Vanuxem, states 

 [1842, p. 35] that Doctor Eights found a rolled stone containing L. acuminata, the surface of the 

 fractured part showing from 60 to 70 valves in an area 2 by 3 inches. 



I have collected this species in situ in great numbers in Saratoga County, and in the valley 

 of the Hudson near Whitehall, in calcareous sandstones probably equivalent to Conrad's 

 "Calciferous sandrock." The shell corresponds to the description given by Conrad, and occurs 

 in great numbers in partings of the rocks. 



In all of the illustrations given of this species from New York, the small, relatively narrow 

 form was figured. The broader and larger form that occurs in the compact, sandy limestone 

 north of Saratoga Springs, and at Beverly, Ontario, does not appear to have been known to 

 the earlier authors. 



It was not until 1863 that the Eastern forms so much like those of "Lingula pinnaformis" 

 of Wisconsin were illustrated by Billings [1863, p. 102]. Matthew [1895b, PL II] illustrated 

 the interior of the ventral and dorsal valves of L. (L.) acuminata from Beverly, Ontario. The 

 muscle scars and their markings, as shown in Matthew's diagrammatic figures, vary from those 



