STATE GEOLOGIST. 109 



margin of the post-abdomen has a series of small spines, and the 

 lower shell-margin is hairy. The post-abdomen of the male is 

 somewhat as in Crepidocercus, and densely hairy; the first foot has 

 a moderate hook. 



To judge from Kurz's statements, P. ornatus, Schoedler, is not 

 specifically distinct. Not yet identified in America. 



Sp. 4. Pleuroxus acutirostris, Birge. 



» 



This form, with Harporhynchus, imitates in some respects the 

 Alonellae, from which they differ in having the beak elongated and 

 recurved. Birge's description does not state what the form of the 

 fornices is, but he intimates that the general resemblances are with 

 Pleuroxus. The general shape is as in P. hamatus. 



''The post-abdomen is broad, compressed, truncated, with numer- 

 ous fine caudal teeth. The terminal claws have only one basal 

 spine." ''The valves are reticulated as in P. [Alonella] insculptus, 

 although not so plainly." Length 0.35 mm. Southampton, Mass. 



Sp. 5. Pleuroxus adunctus, Jurine. 



Monoculus adunctus, jukine . 



Pleuroxus adunctus, schoedler, p. e. MUELiiER, kubz. 



Very like P. trigonellus, but with the back more strongly arched^ 

 The anterior part of the shell is striped. The beak is shorter than 

 in P. trigonellus, but no other permanent differences are discover- 

 able. The temptation to believe this a mere varietal form of P. 

 trigonellus is great. Indeed, four species (the two here noted, P. 

 bairdii and P. denticulatus, Birge,) are very nearly related. The 

 ephippium, where known, is marked by minute punctation and a 

 darker color. 



Sp. 6. Pleuroxus bairdii, Schoedler. 



Pleuroxus trigonellus, baird. 

 Pleuroxus hairdii, kurz. 



This form, so far as can be*gathered from Baird's brief descrip- 

 tion and figures, differs from the others in having the shell marked 

 by straight parallel lines running diagonally backward and upward, 

 and in lacking one of the terminal bristles on the 5-setose ramus of 

 the antennae. The first is a possible but unusual structure, while 

 the second might result from an overlooking of the very small seta 

 which fills this place in the other forms. Baird himself did not 

 distinguish it from P. trigonellus. 



