CAMBRIAN BRACHIOPODA — WALCOTT 73 



Lakhmina Oehlert, Hali< and Clarke, 1892, Eleventh Ana Rept. State. 



Geologist New York for 1891, pp. 234-235. (Described.) 

 Neobolus Waagen, Hall and Clarke, 1892, idem, p. 245. (Described.)^ 

 Lakhmina Oehlert, Hall and Clarke, 1892, Nat. Hist. New York,, 



Paleontolog}-, VHI, Pt. i, pp. 28-30. (Described and discussed.) 

 Xcoboliis Waagen, Hall and Clarke, 1892, idem, p. 84. (Described ani 



discussed.) 1 



General outline of shells broad oval to subcircular; nearly equf- 

 valve, moderately convex. Shell si:bstance calcareo-corneous and 

 probably phosphatic, structure laminated. Surface with concentric 

 striation. Shell strong for its size and built up on its anterior and 

 lateral margins of several thin layers or lamellae. Apex of ventral 

 valve small and slightly projecting over a low false area that appears 

 to have an open delthyrium. Apex of dorsal valve marginal. 



The interior of the ventral valve has a strong rounded central ridge 

 extending from the narrow area about one-third the length of the 

 shell, and a strong ridge on each side that extends from the same 

 point of origin as the central ridge obliquely forward nearly to the 

 frontal margin of the shell. ^ Between the central ridge and the 

 posterior portions of the lateral ridges there are slightly concave 

 shelves forming with the central ridge a triangular platform, with 

 an open space beneath the concave shelves ; numerous radiating striae 

 occur on the concave shelves and the inner surface of the shell. 



Of the muscular impressions in the ventral valve, Dr. Waagen 

 wrote [1885, p. 762] that "nothing can be observed." Considered 

 from the point of view of the Trimerellidae, this may appear to be 

 correct; but if we compare the muscle scars of Obolus with what 

 appear to me to be points of attachment of muscles, there is no diffi- 

 culty in recognizing a few scars. Just beneath the outer extension 

 of the narrow area of the ventral valve there is a minute, clearly 

 defined elongate oval space that corresponds to the divided umbonal 

 muscle scar in Obolus apollinis Eichwald [1829, p. 274]. Near the 

 outer margin, on a line with the anterior portion of the central ridge, 

 there is a narrow elongate space which, under a strong reflected 

 light, is seen to be divided diagonally by a slight, narrow, raised line. 

 Compared with Obolus^ this space is the point of attachment of the 

 transmedian and anterior lateral muscle scars. It is probable that 

 the outside and middle lateral muscle scars and the centrals were 

 attached to the platform, but there are no defined muscle scars 

 upon it. 



^ I do not find any indication of the incurving of these ridges as described 

 and illustrated by Dr. Waagen [1885, p. 762, pi. lxxxv, fig. 6]. 



