Prof. F. M'Coy on new Cambro- Silurian Fossils. 395 



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 strise, 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 strise ; cardinal teeth very short, 

 thick, diverging at 80°. Width 1 inch, proportional length y 7 ^, 

 depth T ^%. 



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 strise 

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

 casts quite different). 



Very abundant in the oolitic limestone and decomposing 



