PRODUCTUS. 37 



Peoductus umbonillatus, King. A Catalogue of the Organic Remains of the Permian 



Rocks of Northumberland and Durham, p. 8, 19th of 

 August, 1848; Monograph of English Permian Fossils, 

 p. 92, pi. xi, figs. 14 — 18, 1850; Anlosteges umbonilla- 

 tus, King, Annals and Mag. of Nat. Hist., vol. xvii, 

 2d series, pi. xii, fig. 6, March and April, 1856. 



More or less sub-quadrate aud concavo-convex, without articulating condyles ; the larger 

 valve is slightly convex, and rather flattened when young (fig. 1) ; more gibbous, and 

 sometimes geniculated, when adult (fig. 4), with a shallow sinus commencing at about the 

 middle of the valve, and extending to the frontal margin. The beak is not much inflated, 

 nor is it produced beyond the extremity of the cardinal edge ; the auricular expansions 

 are hardly defined, the hinge-line being in general shorter than the greatest width of the 

 shell ; the margin is also more or less thickened, so as to simulate a false area, with a 

 narrow uncovered fissure ? The external surface of the valve is ornamented by a small 

 number of irregularly scattered spines, which attain at times a considerable length. The 

 dorsal or smaller valve is very slightly concave, with a small mesial wave. 



In the interior of the ventral valve the muscular impressions occupy a pyriform space 

 at the bottom of the valve ; those produced by the adductor are oval, narrow, and placed 

 on a mesial elevation ; on either side of these are seen the longitudinally striated sub- 

 quadrate impressions left by the cardinal muscle. In the interior of the dorsal valve the 

 hinge-line margin is more or less flattened (figs. 9 and 10) ; the cardinal process, which 

 projects at almost right angles to the plane of the valve, is bifid, with a deep slit or groove 

 along the upper surface of each lobe. Under this a narrow longitudinal ridge extends to 

 nearly two thirds of the length of the valve, and on either side are seen the scars produced 

 by the adductor and reniform impression (figs. 5, 6). The remaining unoccupied surface 

 of the interior is minutely pitted on the posterior half, and covered with produced tubercles 

 on the anterior portion, and especially in the vicinity of the frontal margin. 



The largest British examples I have seen measured 1 2 lines in length by 1 6 in breadth, 

 but the shell is in general of smaller dimensions. 



This species is well distinguished from Productus horridus by its external and internal 

 details, as will be at once perceived from a comparison of the illustrative series of figures 

 given in my PI. IV, and which were drawn from the best examples hitherto discovered by 

 Mr. Kirkby in the shell limestone of Tunstall hill, and by Mr. Howse in that of Dalton-le- 

 Dale, Durham. 1 Specimens in which the shell is preserved appear rare ; but beautifully 

 perfect internal casts are rather more abundant, and upon them, in relief, may be studied 



1 Mr. Howse observes, that "it differs from its congener in several important particulars. The boss 

 or muscular fulcrum, the shape of the muscular impressions, the greater size of the oral arms" (Mr. Howse 

 alludes here to the reniform impressions), " the absence of cardinal spines on the upper valve, the flanging of 

 the hinge-margin of the upper valve, are so strongly characterised, that it cannot be mistaken for any other 

 species." 



