REPORT OF THE STATE PALEONTOLOGIST I903 323 



apical conch in the gentis Endoceras agrees with the form de- 

 scribed, but added that he was able to trace in several species of 

 Endoceras the apical portion to a diameter of a few millimeters, 

 and that in all of them it was simple and conical, and possessed 

 septa and siphuncle like the remainder of the phragmocone. 



In 1894 Clarke described a species with similar apical cone 

 from the Trenton beds in Minnesota, making it the type of a 

 new genus, Nanno aulema [1894, p. 205]. In the Minne- 

 sota report [1897, p. 770] this interesting form has been described 

 very elaborately and it has been pointed out there that " the 

 continuance of an aseptate condition for a considerable period 

 in the early history of Nanno is itself indicative of an important 

 difference from Endoceras (Cameroceras) and Piloceras, inas- 

 much as this determines it to have been a more elementary 

 organism than either." Holm's species is here also referred to 

 Nanno. It is evident that both observers saw in the free apical 

 cone a differential feature of considerable importance. 



On account of Holm's conservative reference of his species to 

 Endoceras, the validity of the genus Nanno was questioned by 

 several authors (Sardeson, Bather). Holm himself discussed 

 the relations of the endosiphonal structures soon after [1895, 

 p.6i6] and came to the conclusion that inasmuch as it is not yet 

 established that the apexes of all species of Endoceras have not 

 the same structure as that of E. b e 1 e m n i t i f o r m e, the only 

 difference between Endoceras and Nanno consists in the unequal 

 longitudinal and transverse dimensions of the siphonal apical 

 cone: the siphuncle of Nanno attaining its greatest width within 

 the apical cone, whence it decreases to the beginning of the 

 cameration, while in the other Endoceratidae the siphonal apical 

 cone began undoubtedly very small, and the siphuncle increased 

 gradually within the chambered conch. For this reason he 

 adopted the term Nanno for a subgeneric group of Endoceras 

 and in the following year (1896) described tAvo additional types 

 of this subgenus, adding also another subgenus Suecoceras. He 

 redefined the subgenus Nanno, seeing its principal diagnostic 

 character in the inflated apical cone which corresponds in length 

 to the combined length of at least three of the oldest cameras,, 

 and which thereafter contracts so rapidly that already within 



