492 
ceras, but are more widely open and often have spreading lips to the 
lateral sinuses as in Ptenoceras (Gyr.) alatum, Barrande, Pl. 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, Pl. xliv, and Ptenoceras 
tardum should also be included in this genus, and probably Bar- 
rande’s Zrochoceras nodosum, P\. xxv, but his other forms described 
under this name belong to widely different genera. 
Hercoceras. 
This genus described by Barrande (Systeme Stlurien, 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 incontact. 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, Pl. viii, shows the small comma-shaped umbilical perfora- 
tion, deep apical chamber and septa of the nepionic stage. The 
czecum 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. 
fTercoceras ( Troch.) transtens, sp. Barrande, Pl. xxx, is a species of 
this genus, and it seems to me quite possible that Barrande’s 
