CAMBRO-SILURIAN FOSSILS. 217 



This species is flatter and less wide than the Atrypa {Penta- 

 merus) undata (Sow.), but less long than the Atrypa (Pentamerus) 

 lens, being intermediate in form between the two. The casts 

 show, however^ in the small valve a rather strong slit in the beak 

 for the short medial septum, and a transverse pit on each side 

 for the origin of the apophyses or hinge-teeth. The receiving 

 valve has two short diverging dental lamellae, bordering its beak 

 as in Hemithyris generally, with one or two small ridges between 

 them ; the sides of both valves show about three straight once- or 

 twice-branched ridges of the pallial and ovarian vessels. 



Very common in the schists and limestone of Mathyrafal S. 

 of Meifod, Montgomeryshire ; and in the schists of Pen y Craig, 

 Llangynyw, Montgomeryshire ; slate of Alt ffair ffynnon, Llan- 

 fyllin. North Wales. 



{Col, University of Cambridge.) 



Orthis Hirnantensis (M'Coy). 



Sp. Char. Truncato-orbicular, depressed ; cardinal area very low, 

 triangular, nearly twelve times wider than high ; hinge-line 

 slightly less than the width of the shell ; cardinal angles 

 slightly obtuse, lateral and front margins horizontal, almost 

 uniformly curved ; entering valve perfectly flat, with a slight 

 longitudinal mesial depression near the beak ; receiving valve 

 slightly and gently convex, most so along the middle, at about 

 one-third the length from the beak ; both valves with nume- 

 rous slightly unequal, prominent, angular, strongly fascicu- 

 lated striae, each of the primary ridges branching near the 

 middle into from five to seven smaller, forming in some spe- 

 cimens slightly angulated, divaricatingly arched groups, eight 

 or ten strise at the cardinal angles smaller and straighter than 

 the rest, running nearly parallel with the hinge-line ; separa- 

 ting sulci narrow, marked with very coarse punctures or little 

 pits, and crossed by coarse obtuse transverse strise ; twelve to 

 fourteen strise in two lines at four lines from the beak ; in- 

 ternal cast of receiving valve radiated with coarsely punctured 

 impressions of the external striae; cardinal teeth very short, 

 thick, diverging at 80°. Width 1 inch, proportional length -^-^-q, 

 depth ^^o'o- 



This is an extremely beautiful species, remarkable for its flat- 

 ness and broad divaricating bands of coarse, branched strise, which 

 are not at all arched along the hinge-line, as in the somewhat 

 similar O. retrorsistria (in which the depth is greater, the striae 

 much more uniform, and the surface smoother, and the internal 

 casts quite difi^erent). 



Very abundant in the oolitic limestone and decomposing 



