94 MOLLUSCA. 



[460, Cast]. Belemnites dilatatus, Blainv. 



GuAKD. This Belemnite has a compressed guard and the longitu- 

 dinal furrow situated on the margin opposite the siphuncle. The 

 form is characteristic of the Neocomian beds. These specimens (three 

 in number) are from the Lower Neocomian (Cretaceou.s), France, and 

 belong to the Ward Collection, University of Rochester. 



No. 177. Belemnites hastatus, Blainv. 



* In this species the conotheca seems to have been extended in a broad, sim- 

 ple plate. From the Oolite, Weymouth, England. 



No. 178. Belemnites densus, M. & H. 



The guards of this species which are quite common in some of the Jurassic 

 beds of the Black Hills region, appear to be very variable in thickness and to 

 be closely related to B. Panderisnus, d'Orb. From Rapid City, Dakota. 



No. 179. Belemnites elongatus, Miller. 

 From Middle Lias, Charmouth, England. 



No. 180. Belemnite , Lamarck. 



Ink-bag. The sepia of the dibranchs is frequently found fossilized, and 

 yet capable of being used as an ink-bag. Some ink-bags are nearly a foot 

 long, From Lyme-Regis, England. 



No. 181. Belemnitella mucronata, Schlot. 



"The guards of this genus are mainly distinguished from those of Belem- 

 nites by their ventral slit and vascular markings. And the genus appears to 

 be entirely confined to the horizon of the true Chalk and the Upper Greensand 

 of English geologists, and has an almost world-wide geographical range, 

 wherever beds of these horizons occur. Like Belemnites, these mollusks seem 

 to have been gregarious in their habits." These specimens are from the 

 Greensand (Upper Cretaceous) of Marlborough, N. J. 



ORDER TETRABRAISrCHIATA. 



The tetrabrancbs liave two pairs of gills ; numerous retractile 

 tentacles devoid of suckers, and a head i-etractile within an exter- 

 nal shell. This shell is a long cone, straight, folded or coiled, and 

 divided into chambers by a series of curved septa, connected by 

 a siphuncle which is central, ventral or dorsal. The animal lives 

 in the outer chamber, but holds connection with the deserted 

 rooms of its dwelling by means of the siphuncle. The shell is 



* When the species is of small size it will be represented by a number of 

 specimens, — in all cases sufficient to illustrate it. 



