594 CAMBEIAN BRACHIOPODA. 



of the dorsal valve is flat and marked midway by a slightly raised triangular space with 

 depressed triangular spaces on each side; what appear to be flexure lines cross the area very 

 much as they do in the area of the dorsal valve of Obolus apoUinis Eichwald. Pedicle tube 

 or foramen of ventral valve small, cyhndroconical. It has been observed in a broken section 

 of the posterior portion of one shell (PI. LIV, fig. 2nX. The external foraminal opening is a 

 little in advance of the extreme end of the beak. 



The muscle scars are more clearly shown in 0. crassa than in any other species of the 

 genus. The transmedians (i) in both valves are near the margin of the area and close to the 

 mam vascular smuses; the outside and middle laterals (1) of the dorsal valve can not be sep- 

 arated, but their position a little in advance and slightly outside of the transmedian is well 

 indicated on the casts of the interior (PI. LIV, figs. 2k and 21) ; the anterior laterals of the ventral 

 valve are shown by figure 2h at j, a little in advance and outside of the transmedian, also in the 

 dorsal valve as small oval scars each side of the median line near the center of the shell; the 

 central scars (h) of the dorsal valve are elongate, oval, and situated on the inner edge of a 

 groove bounding the visceral cavity, about one-fourth of the distance between the anterior 

 lateral scars and the posterior margin of the shell; the central, middle lateral, and outside 

 lateral scars of the ventral valve have not been differentiated; the area (c) in which they should 

 occur (PL LIV, figs. 2g and 2li) is clearly outlined in a number of specimens; the pedicle and 

 umbonal scars have not been observed. 



The main vascular sinuses of the ventral valve are strongly marked and extend forward 

 well toward the front margin of the shell; they vary in form and position in different shells 

 largely on account of the thickness and manner of deposit of the shell substance. The figures 

 2g-j illustrate better than any description I can give the character of the sinuses and the prin- 

 cipal variations as far as known to me. In the dorsal valve the main sinuses extend farther 

 out from the median line than in the ventral, but they are small and inconspicuous when com- 

 pared with the latter or with the strongly impressed grooves on each side of the visceral cavity. 

 The latter grooves rise to give the peculiar ridges having the appearance of frog legs that are 

 so strongly marked in the cast of the dorsal valve of Oholus {Lingulobolus) spissus (Billings) 

 (PI. XVI, fig. 2d). 



Observations. — In a former paper [1886b, pp. 114-116] I have quoted the descriptions of 

 this species by Hall and Ford. With the material now available and our more detailed infor- 

 mation of the genus, it is unnecessary to repeat or to comment on those descriptions or on 

 the slight additions made by myself, except the reference made to Avicula? desquamata HaU. 



Billings [1861b, p. 7] thought that Avicula f desquamata Hall might be referred to Oholella, 

 and Ford [1871, p. 33] referred Orbicula? crassa Hall to the same genus. Later [1878, p. 128] 

 the latter observer placed Avicula? desquamata as a synonym of 0. crassa, saying: 



The species known as Oholella crassa of the Troy beds may also be briefly noticed in this connection. It includes 

 the species already widely known under the name of 0. desquamata from the same locality, this latter, as may be shown, 

 having been founded upon the dorsal valve of the former. The ventral valve is always more acutely pointed at the 

 beak than the dorsal, but beyond this feature there is nothing, so far as I have been able to discover, by which they 

 may be distinguished from each other externally. The surface of each when perfect is both radiately and concen- 

 trically striated. As a rule, however, the imbricating edges of the successive layers of growth are the only markings 

 visible. 



Oholella crassa is one of the large species of the genus. There is no closely allied species on 

 the American Continent, but 0. mohergi, of Sweden, may be considered as such in the Scan- 

 dinavian Cambrian faunas. Some examples of the outer surface of 0. mohergi resemble 0. 

 crassa, but a careful comparison of the form and also of the interior of the valves shows striking 

 specific differences (Pis. LIV and LV). The presence of a well-marked median furrow on the 

 area of the ventral valve and a relatively small pedicle tube are noticeable features in 0. crassa. 



Oholella crassa is very abundant in some of the limestone layers interbedded in the siliceous 

 shales at Troy, New York. It also occurs in the limestone conglomerate of the same region. 

 It is associated with the EUiptocephala asapTioides fauna in the Hudson Valley, and a 

 similar fauna is found in the limestone bowlders of the Bic conglomerate in the St. Lawrence 



