PARALLELISM. 187 



titinae that they had straight forms among their ances- 

 tors and that these forms have a central siphuncle and 

 suture as among nautiloids. The Devonian Goniati- 

 tinae and some of the Carboniferous forms had also 

 gyroceran forms and loosely coiled nautilian forms, in- 

 dicating an ancestry with similar cones, but at these 

 stages the siphuncle is invariably ventral as in the 

 adults. The young of all of the Ammonitinae, however 

 involute the shell may afterwards become, have an in- 

 variably straight or curved cyrtoceran cone in the 

 apical part, and when they come in contact by growth, 

 the first whorl or whorls are equally invariably open 

 coils like the coils of the fourth grade in nautiloids. 

 The fifth kind of shell, the involute-nautilian, follows 

 in precisely similar succession to what it does in the 

 ontogeny of nautiloids. Farther than this the degree 

 of involution increases according to the species, with 

 age, and the amount of this involution is often an im- 

 portant part of the specific diagnosis. 



'Among Ammonitinae one finds at once that there 

 are no orthoceran or cyrtoceran shells except among 

 the large group designated by the author as Bactrites. 

 This genus begins early in the Silurian with shells that 

 are not distinguishable from true Orthoceras except 

 by having the siphuncle in adults and later stages 

 close to the venter. Some of these forms have no 

 bulb or protoconch and have a large scar on the apex 

 as in true Orthoceras, others have a calcareous bulb 

 or protoconch on the apex as in true Ammonitinae. 

 There are also open or gyroceran shells in the adults 

 of the genus Mimoceras which are repeated in the 

 young of Anarcestes and other genera of Goniatitinae 

 figured in my 'Embryology of Fossil Cephalopods. ' l 



I Bull. Mus. Comp. Zool., III. 



