578 NORTH AMERICAN INDEX FOSSILS. 



CLXXXIII. Polorthus Gabb. 



Teredo-like, calcareous tubes, with habit of boring into sand of 

 sea bottom instead of wood. Tubes with transverse septa of two 

 kinds. Bivalve shell of any kind unknown. Cretacic. 

 607. P. tibialis (Morton). Cretacic. 



Septa convex towards the smaller end of tube, perforated cen- 

 trally by an elliptical slit. Just below the terminal series of septa 

 is an annular muscle scar. Towards the larger extremity of the 

 tubes there are one to three transverse septa convex toward the 

 larger extremity of the tube. 



Jerseyan of New Jersey (Vincentown). 



Class Scaphopoda Bronn. 



The scaphopods are marine mollusks whose bilaterally sym- 

 metrical body is protected by a tubular shell, which is generally 

 somewhat curved and open at both ends. The smaller opening 

 permits the expulsion of waste and genital products, while the 

 larger one is the anterior or mouth opening. The concave side 

 of the shell is the dorsal side, and the additions to the shell are 

 made at the larger end. The smaller end suffers by wear and 

 resorption, and in some genera becomes notched or slit. The shell 

 consists of three distinct layers. 



The modern species inhabit mostly deeper waters where they 

 are partly embedded in the mud or sand. Their known range is 

 from 2 to over 2,400 fathoms. The fossil species described prob- 

 ably lived in moderate depths. 



Literature : Papers on the marine Tertiaries elsewhere cited. 



Family DENTALIID^ Gray. 

 I. Dentalium Linnaeus. 



Shell tusk-like, tubular, curved and regularly tapering, with 

 circular cross section and open ends ; surface with strong longi- 

 tudinal ribs {sens, strict.) or with longitudinal striae or smooth; 

 apex entire or with notch or slit of greater or less length. Ordo- 

 vicic ?-Recent. 

 1 . D. (Laevidentalium) martini Whitfield. (Fig. 794.) 



Devonic. 



