34 BRITISH PERMIAN BRACHIOPODA. 



It varies considerably in external shape, but may be said more or less sub-pentagonal or 

 quadrangular marginally ; wider than long in most individuals, but sometimes the reverse. 

 The ventral valve is convex and gibbous, with prominent auricular expansions extending 

 on either side of its somewhat attenuated but inflated and incurved beak : there exists, 

 also, a mesial furrow, commencing at the extremity of the beak, and extending to the 

 frontal margin. The smaller or dorsal valve is convex, and of moderate depth, with one 

 or three undulations upon its surface, viz., a mesial and two lateral ones. The hinge-line 

 is as long, or a little shorter, than the greatest width of the shell. There is no area on 

 either valve, 1 nor articulating condyles. On the external surface of the ventral valve a 

 variable number of long, rather large, and hollow spines are regularly scattered ; on the 

 dorsal valve they are confined to the vicinity of the cardinal edge, where they assume much 

 regularity in both valves, from being arranged in one or two rows in the proximity of the 

 hinge-line ; they also project externally with an oblique angle, and become larger and 

 longer as they approach the lateral margin. 2 The ventral valve is also obscurely ribbed in 

 some specimens, and marked by numerous concentric lines of growth. 



In the interior of the ventral valve (fig. 22) the muscular impressions occupy the 

 larger portion of a pear-shaped space at the bottom of the valve, which is chiefly situated 

 under the cavity of the beak. This space is longitudinally divided by a raised, slightly 

 convex callosity, along the upper surface of which are impressed the ramified dendritic 

 impressions of the adductor, longitudinally divided by a small mesial ridge (a of the 

 internal cast, fig. 20, as well as on the beak of fig. 19). On either side there is a 

 deep and strongly grooved or striated subquadrate impression, which is supposed to be 

 due to the cardinal muscle (b of fig. 20 3 ), the remaining surface of the interior of the 



p. 465, 1855. Quenstedt, ' Haudbuch der Petref.,'p. 490, tab. 39, figs. 26— 30, 1851. Howse, * Annals and 

 Mag. of Nat. Hist.,' vol. xix, 2d series, p. 4A, 1857, &c. To these works we must refer the reader for more 

 copious details. This Productus has also received several names which will be found recorded in the 

 synonyma published by Professors De Koninck and King, and which dates back as far as Hoppe, who, 

 in 1745, was the first to describe the shell. 



1 M. De Koninck, who minutely and carefully describes every particular connected with the external 

 form of this species, states, at p. 160 of his work, that most specimens are deprived of areas, but that with 

 a certain number it exists in a rudimentary state ; it appears to me to be but a thickening of the hinge-line. 



2 These spines are well described by Professor King, at p. 90 of his Monograph. 



3 Beautifully preserved internal casts of this Productus are not of rare occurrence in the shell limestone 

 of Humbleton hill, and of which figs. 19 and 20 are careful illustrations. Fig. 19 represents the same 

 view of a similar hut much less perfect cast, figured by Sowerby under the name of Productus clava, in 

 pi. 560, fig. 5, of the 'Mineral Conchology.' I considered it, therefore, desirable to offer faithful repre- 

 sentations of these, as such had not been done by other authors, and the more especially so as upon them 

 are seen, in relief, all those impressions which in the shell itself would appear in hollow, as in figs. 21 and 

 22, which are also drawn from admirably preserved detached valves, in the collections of Messrs. Howse 

 and Kirkby. The internal details relating to this interesting fossil have been more or less completely 

 described by M. De Koninck, Dr. Geinitz, Professor King, Mr. Howse, and others : but I trust that my 

 series of illustrations may also be considered of some interest, as they have been drawn from the best 



