REPTILIAN AGE. JURASSIC PERIOD. m 



Genus DENTALIUM, Lin. 



Synon. — Tubulus, Dentate, Dentalites, Syringites, &c., of ante-Linnaean authors. 



Dentalium, Likn^us, Syst. Nat. 1740, sec. ed. 64 ; ib. sixth ed. 1748, 75 ; ib. tenth ed. 1758, 785 ; ib. twelfth 



ed. 1768, 1263.— Lamk. Prodr. 1799, 78 ; Syst. An. 1801, 326 ; Hist. Nat. V, 1818^ 341.— Schum. Ess. 



1817, 263.— Blainv. and Defe. Diet. Sci. Nat. XIII, 1819, 69, &o. 

 Etym. — Dens, a tooth. 

 Examp. — Dentalium elephantinum, Lin. 



Shell elongate, terete or angular, smooth, costate or striate; aperture circular; 

 lip simple, entire ; margin of the posterior opening without a fissure. 



The shells of this genus are very similar to those of the allied group Entails, but 

 the latter differ in having a slit or fissure in the dorsal side at the smaller extremity. 

 The genus Ditrupa, one of the marine worms, also secretes a shelly tube, some- 

 times resembling that of Dentalium, though it can generally be distinguished 

 from the latter by having. its sides more or less ventricose near the aperture, while 

 the shell of Dentalium is gradually and regularly tapering from the larger to the 

 smaller extremity. 



The genus Dentalium seems to have made its first appearance during the Devonian 

 epoch. It is also known to occur in the Carboniferous rocks, and ranges through 

 all the succeeding formations. It is well represented in the existing seas, and 

 probably attains its maximum development at the present time. The recent species 

 are usually found in deep seas, where they are said to feed upon Foraminifera 

 and small bivalves. 



Dentalium snbqiindratitin. 



Dentalium ? subguadratum. Meek, Proceed. Acad. Nat. Sci. July, 1860, 311. 



Shell small, thin, regularly and slightly arcuate, very gradually tapering, flattened on four sides 

 so as to present a subquadrangular section, the angles being a little rounded; section of internal 

 cavity circular ; surface apparently without longitudinal or transverse strise. 



Length, about 1 inch ; diameter of larger end, 0.05 inch ; do. of smaller end, 0.02 inch. 



This species is remarkable for its quadrangular form, which gives rise 

 to some doubts whether or not it is a true Dentalium ; though we know 

 of no other genus to which it can be referred. 



Locality and position. — North Platte River, at the Red Buttes, Lat. D^ntaii 

 42° 50' north, Long. 106° 40' west. (Type, No. 677.) subquadrat«m. 



Order Ctenot>railCllia,ta (= Pectinihranchiata). 

 Suborder ROSTRIFERA. 



Family VALVATIDiE. 



Shell small, turbinate or discoidal, provided with an epidermis ; aper- 

 ture with an entire peritreme ; last volution sometimes free at the aperture. 

 Operculum corneous, circular, multispiral, the whorls being provided with 

 a thin elevated margin. 



Animal with muzzle produced ; tentacles cylindrical ; eyes at their outer 



