[mattokw] «ÏUDIES ON CAMBRIAN FAUNAS 169' 



LiNNARssoNiA Belti Dav. rnut. magna n. nnit. (PI. I., Figs la 



and h.) 



A sranll species which diiiers in the position of the muscle-scars, etc. 

 from i. transversa Hartt and others, belongs to this sub-fauna. 



Form orbicular, valves equal, strongly convex. The ventral valve 

 sub-conical, the dorsal valve tlatter. 



The interior of the ventral valve has a semilunar boss in front of the 

 foraminal opening, behind which is a crescentic depression with a 

 (muscle ?) pit at each end ; in front of the boss are two faint converg- 

 ing raised lines inclosing a narrow pear-shaped area, which extends to 

 the middle of the valve ; the front bounding line of this area is extended 

 on each side for the width of the area. This line is supposed to limit in 

 front the muscular area of the valve. 



The interior of the dorsal valve has a median ridge, broadening 

 forward, and with its end at the middle of the valve. On each side 

 are lateral ridges, beginning one-quarter of the length of the shell from 

 the hinge line, and extending nearly as far as the mesian ridge ; the}' 

 also widen, and are as broad at the anterior end as the mesian lidge ; 

 each lateral ridge terminates in a rruscle scar, the front of Avhich is about 

 half way from the hinge to the anterior edge of the valve. On each 

 side of the mesian ridge at its extremity are small, narrow, longi- 

 tudinal pits, that mark the position of small central muscles. Ju.st 

 in front of the hinge is an oblatel}- oval impression, divided in the 

 middle by the mesian ridge ; this impression holds the position of the 

 posterior muscles. Considerably outside of the muscle-bearing area of 

 the valve, runs a circular groove, interrupted on the mesian line and 

 thei-e flexed outward ; this will be the impression of the vascular trunks, 

 whose extremities end al>out one-quarter of the length of the valve from 

 its front. 



Sculpture. — The outer surface of tlie valves is marked by fine con- 

 centric striti', made visilile l)y a lens, and there are seen by the same 

 means, very faint interrupted radiating striae. 



Size.- — Length and width each 3 mm. 



Horizon. — Found both in the limestone bands of the gray beds and 

 in the black siliceous shale ; frequent. 



This species is easily distinguished from X. transversa, Hartt ' of 

 Div. Ic and d by the more circular form, as well as by the narrow 

 mesian ridge ; it is distinguished from Z/. misera Bill ^ bj' the form and 

 by the much heavier mesian ridge. It is distinct from L. taronica 

 Wale.'* b}' the smaller bo.ss of the interior of the ventral valve, and the 



1 Trans. Koy. Soc. Can., Vol. HI,, 8ec. iv., p. 35, PI. v. tigs. 11, Ha to e. 



2 Trans. Royal Soc. Can., Vol. III., Sec. iv„ p. .35, PJ. v. figs. 12, 12n to e. 



* Fauna of the Upper Taconic, Am. Jour. Soi., Vol. XXXIV., p. 187, PI. 1 tigs. 

 18, 18rf to d. 



