part 4] JURASSIC cheoj^ologt : lias. 285 



particularly in the plate-like shape ; and the penultimate whorl- 

 section of Spath's type agrees with the ultimate whorl-section o£ 

 this genus, suggestive perhaps of common ancestry ; but the pre- 

 sent genus shows no ^^eripheral sulcation, and has a decidedly 

 different radial line. 



Remarks. — Spath says that his genus is ' probably .... far too 

 comprehensive.' ^ It is, therefore, inadvisable to enlarge it for the 

 reception of this form. 



Leptaleoceras leptum, sp. nov. (PL XXVI, figs. 1 a-S b.) 



Description. — Serpenticone ; substeno-leptogyral ; subextre- 

 inilatambilicate ; peripher}^ subtabulate, joining a stout fastigate 

 carina with only a slight angle ; gammiradiate ; lateral area smooth 

 to costulate to smooth. 



General details. — The flat, little-overlapping whorls give to 

 the species a disc-like appearance. The inner whorls are somewhat 

 elliptical in section, the outer whorl is suboblong, thickest about 

 the middle, sloping slightly to the periphery, more to the umbilical 

 edge, which is tabulate, narrow and defined in the last whorl, but 

 indistinct previously. The inner whorls are smooth until about 

 14 mm. diameter ; then furnished with subobscure, undulate costulse 

 which die away nearlj'' to smoothness at about 53 mm. diameter — 

 there remain only occasional traces of the previous costulf© w^ith 

 fine growth-lines, and ultimately only the latter. 



Remarks. — Three specimens are before me, all very similar; 

 but the medium-sized one seems to differ in thickness and slightly 

 in other proportions from the holotype at the same diameter. 



The likeness of the smooth anagenetic stage to the similar stage 

 in Arnioceras is noticeable (see p. 268). 



Distinction. — -The considerable Italian literature on Domerian 

 Hildoceratidse has revealed neither to Mr. Lang nor to myself 

 anything really like this species. I am much indebted to Mr. Lang 

 for kind assistance in bibliographic matters in connexion with tliis 

 Ap|)endix — an important aid at the present time when facilities 

 are so much curtailed. 



Below I give proportions of the examples of this species and of 

 others which have some approximation. Of these Ammonites 

 iioldinqi seems to be the only one possibly of this genus ; but its 

 radial line is perhaps too curved : it would represent the smooth 

 or almost smooth anagenetic stage in this or a parallel line ; A. 

 pseudoradians has the right st\de of radial line, but a different 

 style of ribbing, and shows no sign of the smooth anagenetic stage ; 

 A. normanianus has the style of ribbing, but much too curved 

 a radial line ; and the others differ b}'- being carinatisulcates, with 

 also a too-much flexed radial line. 



1 Q. J. G. S. vol. Ixix (1913) p. 580. 



