M ETAZOA MOLLUSCA . 253 



has pointed out that this change in structure indicates 

 a change in habits whereby a swimming type of Cepha- 

 lopod has been converted into a crawling type. 



In Dactylioceras (No. 604, D. commune Hyatt), the 

 whorls are closely coiled and in the same plane ; all are 

 visible and the ornamentation is similar. 



These characters are seen on a large scale in Ammo- 

 nites parkinsoni which, on account of its size, is placed at 

 the back of the Section (see No. 605 ; No. 606, horizon- 

 tal section of the same). A portion of the shell has been 

 removed in No. 605, and the surface polished, thereby 

 bringing out the sutures finely. In No. 606 the cham- 

 bers and their walls are well shown. 



Asteroceras ( = Ammonites, also = Arietites) obtusum 

 Hyatt (No. 607) has the whorls in one plane and all 

 can be counted. The shell is tightly coiled and even the 

 youngest whorls are ornamented with ribs. This speci- 

 men has most of the shell well preserved. The sutures 

 are hidden, but where the shell is broken off the serrated 

 or fluted edges of the septa are visible though more clearly 

 seen in No. 608 where the outer shell is entirely gone. 

 These sutures are complex as compared with those of 

 Goniatites, Ceratites, or the primitive Ammonites. The 

 siphuncle is seen on the edge and the specimen also 

 shows how the dorsal side of the shell has become im- 

 pressed by close coiling. The living chamber extends 

 backward a considerable distance (No. 608), the last 

 formed septum marking its posterior limit. 



The complexity of the septa is seen in Lytocera s jurense 

 Sitt. (No. 609), where the edges of the septa have been 

 painted red to bring out their structure more plainly. 



No. 6 10, Stephanoceras tarA^&xr/d'Orb., shows the tend- 

 ency of a closely coiled shell to become involute. The 

 last whorl spreads out laterally and partially covers the 

 preceding whorls, leaving a deep umbilicus on either side. 

 The dorsum is deeply impressed in this genus. Several 

 of the specimens in No. 608 have a portion of the exter- 



