210 CAMBRO-SILURIAN FOSSILS. 



Pseudocrania (M'Coy), n. g. 



Gen. Char. Shell slightly inequivalve, free, both valves regularly 

 depressed, subconical, unattached ; dorsal valve with or without 

 a small cardinal area ; internally, margin broad, flat, smooth 

 or minutely striated concentrically, anterior pair of muscular 

 impressions much larger and more strongly marked than the 

 posterior pair ; pallial impressions numerous, linear, not in- 

 terrupted along the middle. 



This palaeozoic genus differs from the true Crania in the fol- 

 lowing points : 1st, Crania is attached by the substance of the 

 dorsal valve, and exhibits thereon an irregular scar ; both valves 

 are free and regular in Pseudocrania-, 2nd, in Crania the pos- 

 terior or marginal pair of adductor muscles are always larger and 

 deeper than the medial or anterior pair ; the reverse is remark- 

 ably the case in the present genus, which also has a smooth or 

 minutely striated margin, destitute of the strong granulation 

 and punctures of most Cranice. The Crania antiquissima as given 

 by Verneuil may be taken as the type of the genus, as also the 

 following species. 



Pseudocrania divaricata (M^Coy). 



Sp. Char. Longitudinally oblong, length and width almost equal, 

 slightly narrowed posteriorly; hinge-line straight, a little 

 shorter than the width of the shell ; dorsal valve (analogue of 

 the lower or attached valve) with a very low triangular cardinal 

 area ; external surface ornamented with sharp, prominent, nar- 

 row rugged lines divaricating from the beaks, those of the 

 sides arched backwards towards the cardinal angles, about 

 equally distant throughout from the intercalation of shorter 

 strise as the longer approach the margin, intervening spaces 

 flat, about four ridges in the space of one line ; internal casts 

 show a broad, flat, defined rim with minute irregular con- 

 centric striae, and the following parts in relief (depressions in 

 the shell) : 1st, a small oval mesial space originating near the 

 hinge, and reaching to one-fourth of the length ; 2nd, on each 

 side of this, close to the hinge-margin, are two diverging oval 

 muscular impressions; 3rd, at a little behind the middle of 

 the shell are two ovate subtrigonal anterior muscular impres- 

 sions considerably larger and deeper than the posterior pair ; 

 4th, close in front of those a narrow semicircular impression 

 (scar of visceral aponeurosis ?) with its extremities arching 

 backwards round the muscular impressions ; 5th, between this 

 and the front and lateral margin, a series of from fifteen to 

 twenty-two uninterrupted equidistant longitudinally narrow 

 pitted (? pallial) marks, each dividmg at its anterior half, 



