SPIRIPERIDJE. 



117 



valve slightly convex, with a depression along the middle. Surface smooth, marked only 

 by concentric lines of growth. Interior unknown. 

 Length 5, width 4, depth 2^ lines. 

 Obs. Only one imperfect specimen having been found (by Mr. Haswell, in the 

 Wenlock Shale of Deerhope Burn, Pentland Hills), I am much puzzled in which of two 

 or three genera it should be located, and I have provisionally left it with Meristella ; for, 

 although several species of Merista, with the arched or shoe-lifter process occur in the 

 Silurian rocks of Bohemia, I am not acquainted with any species of the genus in our 

 British Silurian rocks, unless the small shell, PI. XXI, figs. 28 and 29, may be referred 

 to it. 



Meristella? crassa, /. de C. Soio. (sp.). PI. XIII, figs. 1, 2, 3. 



Atrypa crassa, Sow. Sil. System, pi. xxi, fig. 1, 1839. 

 Spirifera percrassa, M'Coy. Brit. Pal. Foss., p. 194, 1852. 

 ? PoRAMBONiTES CRASSA, Morris. , Cat. Br. Foss., p. 143, 1854. 

 Atrypa? — Salter. Siluria, 2nd edit., pi. ix, figs. 6, 8, 1859 ; Mem. Geol. 



Surv. Great Brit., p. 363, 1866 ; Spirifera percrassa, ibid., 



p. 276. 



Spec. Char. Longitudinally or transversely oval, somewhat sub-rhomboidal, widest 

 about the middle ; anteriorly the sides present a gentle inward curve, while the front is 

 narrowed and slightly rounded ; posteriorly the sides form broad convex curves, and the 

 beak is small and incurved. Valves nearly equally convex (?) and smooth ; a very slightly 

 rounded lobe exists near the front in the dorsal valve, to which corresponds a small 

 depression in the ventral valve. The interior of the latter is considerably thickened ; 

 while from under the extremity of the beak the dental plates diverge widely for a short 

 distance on either side of the beak, and end anteriorly with a hinge-tooth, thus leaving 

 between them a rather deep fissure, of which the sides slope inwards to unite along the 

 bottom of the beak. On leaving the dental projections, these plates converge for some 

 distance to diverge again, until they reach about two thirds or more of the length of the 

 valve, so that towards the middle of the shell they partly surround the muscular area. 

 These dental plates have not a great elevation, but are wide at their base. The lateral 

 portion of the valve, between the dental plate and the margin, is exceedingly thick, and 

 is marked by several obliquely indented grooves, which exist also, but with less depth, on 

 the surface of the interior of this valve that is not occupied by the dental plates and 

 muscular areas. In the interior of the dorsal valve a small hinge-plate is divided by a 

 narrow groove ; under this a slightly elevated median septum extends to about half the 

 length of the shell, dividing the muscular scars, which fonn a pair on either side of the 



