PTEROrODA. 



117 



CLASS PTEROPODA. 



Tliese small eeplialoiis niollnsks are named fi'om the resem- 

 blance of their cliief organs of motion to a pair of wings. They 

 are either naked, or provided with a delicate translucent sliell. 

 In their first stages, they exactly resemble the gasteropod fry, 

 and accordingly Lamarck, De Blainville and Owen regard them as 

 a snb-class of the "•crawlers"; but CuVier, Woodward and Trj'on 

 give them a higher rank. The shell, when existing, resembles 

 either a univalve or a bivalve in which tiie two valves have been 

 cemented along the hinge. These " sea-butterflies," as they have 

 been called, float in mid-ocean, forever out of sight of land, and 

 are the food of northern whales and sea-birds. 



There are several hundred fossil species, some of them appear- 

 ing: in the earliest Silurian or in the Cambrian, and being char- 

 acteristic of the Silurian and Devonian. 



No. 307. Conularia Trentonensis, Hall. * 



This is one of the large,st and most extraordinary of the 

 Pteropods. The shells are sometimes nearly a foot long. 

 They are distinctly four-sided and finely striated with trans- 

 verse lines. They occur in the Mesozoic. but are character- 

 istically Palaeozoic, beginning in the earliest Silurian. About 

 one hundred species are known. Trenton Limestone, Trenton 

 Falls, N. Y. 



No. 308. Tentaculites irregularis, Hall. 



This genus has been referred to the tubicular annelids, and its 

 true position is still in question. It resembles the annelid Cornulites. 

 The shell is a thm, straight conical tube, with numerous annulations 

 or thickened rings. They are of small size, from a couple of lines 

 to an inch in length. They occur in the Devonian, but are chiefly 

 Silurian. In some strata the tiny shells are found in myriads. 

 This slab, covered with specimens, is from the formation to which 

 they have given the name Tentaculite Limestone (Upper Silurian), 

 Schoharie, New York. 



*It may be necessary, in making up collections, to occasional! j^ substitute 

 some other species or the same species from a different locality than the one 

 called for. 



