Prof. W. King's Azotes on Permian Fossils. 267 



directed forward on tlic back, erect and bent backward on tlic 

 sides and adjacent to the hinge; both inner and (?) outer surface 

 marked with nunjcrous tine striix; radiating from the umbone; 

 also with well-drtined incremental lines : uinbone somewhat 

 tumid ; occasionally a little impressed, and incurving over or 

 below the cardinal edge : area small ; scarcely perceptible through 

 the incurvation of the umbone : teeth well developed. Small 

 valve slightly concave ; marked with numerous elongated in- 

 dented impressions, which are somewhat regularly arranged, and 

 deepest at their anterior end ; both inner and outer surface 

 marked with distinct radiating stride; those on the sides near 

 the hinge dichotomous and arcuated or curving posteriorly ; 

 they pass uninterruptedly over the indented impressions *, and 

 are crossed by rather strong incremental lines : nucleus raised a 

 little above the general surface of the valve. 



I have adhered closely to the order in which the various cha- 

 racters of S. Morrisiana are described, so that the differences 

 between the two forms may be readily seen. <S'. Cancrini differs 

 from the present one, — notably in having the large valve still 

 more convex ; the umbone pointed and more swelled out behind 

 the hinge-line ; and the lateral wrinkles transversely directed ; 

 also in having an imperceptible or concealed area, the width of 

 which, too, appears to be greater : — furthennorc, it seems to be a 

 nnxch smaller species, — var. Humbletonensis being the largest of 

 the three forms : some sj)ecimens of the latter are nearly an 

 inch and a half in diameter. De Koninck's figure of his so-called 

 Productus Cancrini from Ust-Josehuga would lead one to sup- 

 pose that its small valve was furnished with spines; but I am 

 almost certain that such do not occur in the present fossil : 

 probably the singular elongated indented impressions maj' be 

 modified bases of abortive spines similar to those on the corre- 

 sponding valve of Productus horridusf. 



It is this variety which exhibits the remarkable trivalved 

 character noticed elsewhere J. I am still at a loss to account for 

 it satisfactorily. The extra valve belongs to the small valve. 

 Can the mollusk of some specimens have had the mantle belong- 

 ing to its small valve separated into two layers or divisions, the 

 innermost one forming the inner valve, and the outermost the 

 outer valve ? 



I suspect all the specimens described and figured by Geinitz, 

 as Orthothrix lamellosus and Productus Cancrini, belong to 

 var. Humbletonensis. Schauroth represents a specimen from the 



* These striae I formerly described as broken : I was misled by the in- 

 dented impressions giving them an appearance of the kind, 

 t Vide Monograph, p. 90. pi. 11. tigs. 6, /• 

 X Vide Monograph, p. 101. pi. 12. figs. 21-2-1. 



