part 4] THE EVOLrTIOlS^ of LIASSIC &ASTEOPODS. 



321 



closely-placed axials. Oral margin not complete in the holotjpe, 

 but in a topotype it shows a distinct sinuosity anteriorly. 



Development. — This form has a smooth embryonic whorl, 

 followed immediately by a stage with one spiral forming a carina ; 



by the following whorl a second 



Fig. 11. — Procerithium tenui- 

 ornatum, sp. nov. HoJotype 

 x5. 



spiral has appeared, which in the 

 course of the next two whorls 

 becomes almost equal in strength 

 to the first; from this point on- 

 wards to the eighth Avhorl, the 

 whorls are flattish and feebly 

 bicarinate, the spirals being near 

 the sutures. Meanwhile, fine 

 slightly curved axials have ap- 

 peared in the space between the 

 two primary spirals, and several 

 finer secondary spirals have been 

 intercalated. By the ninth whorl 

 the two median spirals, which 

 are fairly close together, have 

 become at least equal in strength 

 to the two primaries (and bear 

 tiny tubercles where they are 

 crossed by the axials), and give to 

 the shell a slightly convex ap- 

 pearance. In the remaining 

 whorls these four spirals are still 

 prominent, while other finer 

 spirals are intercalated. There 

 is considerable variation in the 

 spacing of the axials in the 

 holotype : in the first few whorls 

 the axials are strong, and on the eighth whorl are only twenty in 

 number; there is a rapid change, however, on the following whorl, 

 the coarse axials giving place to some fifty fine striae ; on the 

 succeeding whorl again there is a return to coarse ornament, while 

 a portion of the eleventh whorl has fine striations, and on the last 

 whorl fine and strong axials are to some extent alternate. The 

 aperture is not seen in the holotype ; but, if we judge from topo- 

 types, it would be approximately subquadrate. The flatness of the 

 whorl is probabl}' due, as we have noted in several more primitive 

 species, to the prominence of the two primary spirals. 



In Mr. L. Kichardson's collection are numerous specimens 

 approximating to this form, but with many variations in detail 

 (for instance, the strength of the axials and the position of the 

 accentuated spirals) ; all are alike, however, in their fine ornament, 

 and in their flattish whorls showing some degree of bicarination in 

 development. 



This species is probably closely related to Proceritliium foveo- 

 latum (Tate). That writer in his description drew attention to the 



[Lower Lias (armatus zone), Folly 

 Lane Brickworks, Cheltenham. 

 L. Richardson coll. L.Gr. 62.] 



