624 



MOLLUSCA 



PHYLUSr VI 



paragerontic substage, but the condition is in no sense phylogerontic except 



Lohites, and the like (Figs. 1217, 1218). 



Pompeckj, in an important essay, asserts that contracted living chambers 



are invariably developed in old age, and that 

 small shells possessing them are consequently not 

 immature individuals, but dwarfs (Fig. 1156). 

 It is probable that large numbers of shells are 

 indeed dwarfs, but it is also a fact that contrac- 

 tion of the living chamber and volutions occurs 

 in some forms during comparatively early stages ; 

 and sometimes in such a way as to affect the 

 ephebic stages of the ontogeny, when the forms 

 become truly phylogerontic. This latter term is 

 used to designate shells in which the ontogeny 

 has become permanently modified by the assump- 

 tion of retrogressive characters that were intro- 

 duced first in the senile stages of allied pro- 

 pjy ^^5^ gressive species. Whether these peculiar forms 



sciiioenhachia cristata (Deiuc). have contracted apertures in their earlier stages, 



Gauit. Aperture with ventral ^^^^ ^^^jj resorb them before building further, or 



rostrum. o ' 



whether they never add lateral lappets, rostra, 

 etc., as claimed by Pompeckj, until the last resting stage of the ontogeny 

 (Fig. 1156), it is obvious that they are permanently affected by phylogerontic 



Spliaeroceras tironcj- 

 niarti (Sowb.). Oolite. 

 Apertui'e with bi-oad, 

 contracted ventral ros- 

 trum. 



Fin. 1154. 



Normann ites hraiken- 

 ridijl (Sowb.). Oolite. 

 Apei'ture with lateiul 

 lappets. 



FiiJ. 1155. 



Oppclia nimhata 

 (Oppel). Jura. 



Latei'al lappets. 



Fio. 1156. 



Oecoptychius re- 

 fractus (de Haan). 

 Jura. Living chamber 

 contracted, with ros- 

 trum and lappets. 



characters. These forms are comparatively rare in the Trias {Loldtes, 

 Cochloceras), but their number is sensibly increased in the Jura, although 

 usually confined to special localities. During the Cretaceous they become 

 more numerous and more widely distributed (Figs. 1261, 1262). In their 

 extreme modifications they become more or less uncoiled and finally per- 

 fectly straight. 



Crick and Waagen maintain that Ammonoids had an annular band as well 

 as shell muscles, and that these served both to hold the animal in the living 

 chamber, and also formed an air-tight band around the face of the mantle, 

 fastening the latter to the shell (Fig. 1157). Such was, however, probably 

 not the only means of attaching the animal to the shell. The steady pro- 

 gressive complication of sutures, affecting both lobes and saddles as well as 



