344 ME. L. E. SPATH OX THE [Dec. 1914, 



The specimens are still perfectly smooth at a diameter of 5 mm., 

 though, after this diameter is reached, some of the casts begin to 

 show shallow, sigmoidal depressions on the sides, corresponding to 

 indefinite pleats on the original shell. 



The constrictions of the young shells, however, are an important 

 feature. They appear sometimes as soon as the fifth, more often 

 at about the seventh to tenth septum, and persist fairly regularly 

 on the innermost two and a half or three whorls. Those specimens 

 that show twelve constrictions up to a diameter of 35 mm. belong 

 exclusively to the lower horizons (7 and 10 feet above the Belem- 

 nite-Stone on Black Yen) ; whereas in the upper beds (35 feet 

 above the Belemnite-Stone on Stonebarrow Cliff = horizon of the 

 Upper Limestone), seven or eight constrictions persisting to about 

 the diameter of 25 or 2 75 mm. were observed. Since, however, 

 there is a good deal of variation in the number of constrictions, even 

 in the specimens from the lower beds, and since the shells of un- 

 doubted Upper Limestone horizon are comparatively few, it is 

 uncertain whether any phylogenetic significance attaches to this 

 difference. 



Figs. 2 a & 3 a in PL XLIX show the course of the constrictions. 

 They are slightly inclined forward, except, perhaps, the first one, 

 and follow one another at intervals of, generally, a fifth to a sixth 

 of a whorl. 



The smallest specimens that have the shell preserved (9 and 

 15 mm. in diameter respectively) show the costation of the adult 

 form apparently a little more pronounced. This, however, applies 

 chiefly to the middle of the side, where also the previously-mentioned 

 sigmoidal grooves on casts exceeding 5 mm. in diameter appear. 

 There is no tendency to ornamentation manifest near the peri- 

 phery. The curve described by the radii is shown in figs. 6-14 of 

 PL L, together with the course of the striation in typical forms of 

 Monophyllites, Mojsvarifes, JPsiloceras, JEuphyllites and Bhaco- 

 pJiyUites. 1 Several body-chamber fragments from higher horizons, 

 up to and including the lower part of the zone of Amaltliev.z 

 margaritcrfits, show a striate ornament with very distinct fascicu- 

 lation ; whereas the usual specimens, like SoAverby's type, show a 

 more or less irregular, lineate ornament. Occasionally, very tine 

 striation is superimposed upon the lineation or costation ; but, in 

 typical forms, the cost* do not blend into thickened bundles of 

 radii near the inner part of the side. No definite order in the 

 variation of ornament and in the occasional sudden appearance of 

 scales on the periphery could be demonstrated ; neither could it 

 be determined as of zonal significance. 



Three small specimens (13, 14, & 17 of Table I) and four 

 other fragments, unfortunately not found in place, deserve special 

 mention, on account of differences in ornament. Two of the 

 fragments and specimen 17 are distinguished by close sigmoidal 



1 Only one specimen (a body-chamber fragment from Watford, near Rugby. 

 B.M. 20138) was found to differ in the curve of the radial line, haying 

 the rostration near the periphery unusually pronounced. 



