250 SYNOPTIC COLLECTION. 



AMMONOIDEA. 



There are sufficient remnants of the early transitional 

 forms left in the Silurian rocks to demonstrate the former 

 connection of the Nautiloids and the Ammonoids. 1 The 

 common ancestor for the two groups probably existed in 

 pre-Cambrian times, but this form has not been discov- 

 ered. Clarke 2 has described an Orthoceras-like form 

 from the Devonian. It has a large, plump protoconch 

 (PI. 598, fig. i) like that of Ammonoids, but the central 

 position of the siphuncle (fig. 2) is like that of Orthoceras. 

 According to Clarke this protoconch was probably de- 

 rived from so young a shell that atrophy and wrinkling 

 had not taken place, as is the case with the mature Ortho- 

 ceran shells, all of which have been found in a later geo- 

 logic formation. The connection of the protoconch with 

 the conch is seen to be large, which is the case with the 

 generalized Ammonoids, while it is narrow in the Nauti- 

 loids, as seen in PI. 586. 



Bactrites is probably the primitive straight form from 

 which the Ammonoidea arose. The protoconch is seen 

 in PI. 599, fig. i, while in fig. 2 it occurs with a few 

 chambers. 



When this protoconch is not present, the apex (fig. 3 ) 

 is marked by a scar (figs. 3, 4). When the protoconch is 

 broken off the opening of the siphuncle in the first sep- 

 tum (PI. 599, fig. 5, somewhat broken), is seen to be intra- 

 marginal instead of being central as in Orthoceras, or 

 marginal as in typical Ammonoids. PI. 599, fig. 6, shows 

 the protoconch and the enlargement of the shell which 

 goes on until two chambers have been formed, after 

 which the tube contracts. 



An older specimen is figured in PI. 599, fig. 7 (a dorsal 



1 Hyatt, Proc. Amer. Assoc. Adv. Sci., XXXII, 1883. 



2 Amer. Geol., XII, 1893, p. 112. 



