220 CAMBRO-SILURIAN FOSSILS. 



Or this sagittifera (M'Coy). 



Sp. Char. Rotundato-quadrate or oblong, lengtli usually a little 

 greater than the width, depressed ; hinge-line as wide as the 

 shell ; receiving valve obtusely subcarinate along the middle, 

 most so near the beak, which is projecting, pointed, very 

 slightly incurved, apical angle 140°, profile only slightly 

 curved, lateral margins horizontal or with a slight mesial 

 wave towards the receiving valve ; cardinal area of receiving 

 valve concave, triangular, four times wider than high, inclining 

 backwards at 120° , triangular foramen narrow, open to the 

 apex ; entering valve gibbous, deeper than the greatest depth 

 a little behind the middle, profile strongly arched ; cardinal 

 area very narrow, about one-third the height of the other, to 

 which it is nearly at right angles, or in the plane of the mar- 

 gin ; beak small, depressed, with a deep narrow mesial sulcus 

 extending from it about halfway to the margin ; surface of both 

 valves radiated with very close, numerous, obtuse, subequal, 

 irregular, branching striae, separated by much narrower, deep, 

 coarsely punctured sulci, about nine in two hues at six lines 

 from the beak, straight in the middle, and along the hinge- 

 line, slightly arched divaricatingly and most branched at the 

 sides, no fasciculation ; internal cast of receiving valve without 

 mesial septum, two short dental lamellae diverging from the 

 beak at 80°, forming part of the lateral boundaries of a short, 

 prominent, flattened, triangular pair of muscular impressions, 

 slightly wider than long, scarcely one-third the length of the 

 shell, usually tripartite by a pair of wide longitudinal sulci, 

 rest of surface finely radiated by impressions of the external 

 strise ; cast of entering valve with the triangular boss of the 

 foramen, slit by a very slender rostral tooth, from which a 

 narrow obtusely angular furrow (corresponding with the ex- 

 ternal one) extends about half the length of the shell ; car- 

 dinal teeth forming long deep sulci, rather more than one- 

 fourth the length of the shell, and diverging at 60°, resem- 

 bling the mark of a broad arrow-head, of which the mesial 

 sulcus was the shaft, surface impressed by the external radiating 

 strise. Width (of small specimen) 8 lines, proportional length 

 of receiving valve -f-^-Qy of entering valve -f-^^, depth of receiving 

 valve about y^^^, depth of entering valve varying from y^^^^ to 

 •Y^Q' Length often upwards of 1 inch, the length and depth 

 greater in proportion to the width in these old specimens. 



This species is perhaps most nearly allied to the O. turgida 

 (M'^Coy), from which it is distinguished externally by its flatter 

 receiving valve with nearly straight profile, and a cardinal area 

 less than half the height, and a much more obtuse apical angle ; 



