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ceras, but are more widely open and often have spreading lip? to the 

 lateral sinuses as in Ptenoceras (Gyr.) a/atum, Barrande, PI. xliv. 



These forms are interesting because of their obviously close 

 genetic affinity with Hercoceras, and yet the entire absence of an 

 impressed zone at any stage in consequence of the loose mode of the 

 coiling. 



Ptenoceras flexum, sp. Barrande, PI. xliv, and Ptenoceras 

 tardum should also be included in this genus, and probably Bar- 

 rande' s Trochoceras nodosum, PL xxv, but his other forms described 

 under this name belong to widely different genera. 



Hercoceras. 



This genus described by Barrande (Systeme Silurien, ii, Text i, 

 p. 152) should be limited to such species as those placed under this 

 name by this author. I formerly included also under this name cer- 

 tain gyroceran and trochoceran forms. These are separated here, 

 but so far as regards their near affinity, I still hold the opinion that 

 they belong to the same family group, and are genetically connected. 

 Hercoceras includes only nautilian forms, having a small umbilical 

 perforation, the impressed zone being present only after the whorls 

 come in contact. They have peculiar contracted apertures, figured 

 by Barrande, depressed whorls broader on the venter than on the 

 dorsum, often with large spines or nodes, siphuncle subventran, 

 and are often trochoceran in their mode of growth. 



The section of Hercoceras mirum, the type of this genus, given in 

 Figs. 13, PI. viii, shows the small comma-shaped umbilical perfora- 

 tion, deep apical chamber and septa of the nepionic stage. The 

 caecum is very small and is packed away in the ventral angle of this 

 chamber under the septum. The siphuncle is phenomenally small 

 in this genus in the nepionic substages, but increases subsequently 

 to a respectable size. This and the absence of longitudinal ridges 

 on the exterior of all of this genus and its allies has a genetic sig- 

 nificance which is not yet understood. 



The young are sometimes in contact only in the neanic stage, and 

 in the same species this may vary so that the whorls remain in con- 

 tact throughout the ephebic stage, the last whorl with the living 

 chamber being free as in Hercoceras (Gyroc.) nudum, sp. Barrande. 

 Hercoceras (Trock.) transiens, sp. Barrande, PL xxx, is a species of 

 this genus, and it seems to me quite possible that Barrande's 



