MUSEUM OF COMPARATIVE ZOOLOGY. 7-3 



of the whole order passes its climax in the lower and middle Jura, we 

 find the development of a whole group affected by this retardation, and 

 the spiral is common to several generic forms. 



The ovisac of Goniatites crenistria differs from that of the Ammo- 

 nites in the greater breadth proportionally of the abdomino-dorsal axis. 

 The ovisac of G. primordialis, G. retrorsus var., and G. diadema do 

 not seem to differ in this respect. Even the two depressions and the 

 bulging of the sides are as well marked as in the ovisac of Deroceras 

 planicosta.* 



The ovisac of Nautilus was not present in any of the seven speci- 

 mens examined by me. The form of the mouth of this, however, can 

 be inferred from the oval ridge on the ajiex of the first whorl, and 

 the central scar which marks the former aperture through which the 

 animal probably passed into the fundus of the first whorl.f The outer 

 limits of the area thus marked out are flask-shaped. The lower portion 

 or abdomen, if it were extended laterally, would correspond to the 

 broader abdomen of the Ammonoidal ovisac, and the depressions on 

 either side of the dorsum to the embryonal umbilici. The apex of the 

 whorl rises in a well-defined ridge which marks out this area on the 

 dorsum and the sides. At the abdominal extremity the defining ridge 

 is hardly distinguishable, and the shell rises directly to the cicatrix, 

 which is here the most elevated portion of the apex. The striae of 

 growth and the longitudinal furrows both cross this area, but are 

 arrested at the edge of the scar. The ridge, -when seen from the side, 

 is found to be accompanied by a narrow, shallow, slightly concave 

 band, which at the dorsal end is particularly well marked. The edges 

 of the cicatrix, which is a flattened, corrugated, elongated narrow space, 

 are tumid and more or less elevated. 



The ridge appears to mark the line along which the extreme outer 

 edge of the mouth of the ovisac abutted against the apex of the whorl. 

 The lips of the embryo shell were probably inflected as in Gompho- 

 ceras, but instead of being convex as in that genus, were probably 

 concave. The projecting edge of this concave area would then have 

 fitted neatly into the shallow channel or area running around the inner 

 side of the ridge, and the concavity have been the mould upon which 



* Plate III, Figs. 3, 4, 5. The ovisac differs more when viewed laterally, since the 

 outline of a section through the centre, as in Fig. 5, is more decidedly circular, 

 t Plate III, Fig. 1, and ideal section last page of the text. 



