244 SYNOPTIC COLLECTION. 



TETRABRANCHIATA. NAUTILOIDEA. 



When the Nautiloidea first appear in the Palaeozoic 

 strata they are specialized in so many ways, the inference 

 may be drawn that we are ignorant of the radical stock 

 from which all the Cephalopods have descended. A study 

 of the embryological and larval structure of living Nauti- 

 loids, however, leads to the conclusion that Diphragmo- 

 ceras, Piloceras (PI. 583), and Endoceras (No. 584) are 

 among the generalized ancestors of the group. A knowl- 

 edge of these genera and of the Orthoceratites, soon to be 

 described, enables one to give an hypothetical ancestral 

 form with a great degree of certainty. 



Diphragmoceras is a nearly straight tube divided into 

 a few chambers by simple plain partitions or septa. 

 Through these septa runs a large tube or siphuncle which 

 is divided by septa similar to those in the surrounding 

 shell. The structure of the siphuncle is an important 

 character in the classification of the Nautiloids, primitive 

 forms and the young of specialized genera having large 

 siphuncles, while those of adult specialized forms are small 

 and contracted. 1 



Unfortunately the young Piloceras has not been de- 

 scribed. The adult is both straight (PL 583) and curved. 

 It is divided by septa and has a large siphuncle. Near 

 the apex the siphuncle exhibits a cone-in-cone structure 

 which is more developed in Endoceras. The latter genus, 

 when young, may represent the full grown Piloceras. 

 The adult Endoceras (No. 584; PI. 585, vertical section 

 of E. proteiforme Hall, greatly reduced), is a straight form 

 with a shell divided into chambers by septa. Each septum 

 extends downward beyond the one next below, forming 

 funnels which taken together make a complete siphun- 

 cle (see PI. 585). This tube is large, being sometimes 



1 See Hyatt, Proc. Amer. Assoc. Adv. Sci., XLVII, 1898, p. 363. 



