PARARHYTIDA. 53 



Under this generic head may be comprised two groups, as 

 follows : 



Section Pararhylida s. sh: 



Shell thick lens-shaped, in form like Trochomorpha. Whorls about 

 6, slowly increasing, acutely keeled, basal lip somewhat sinuous. Type 

 H. didyodes Pfr., pi. 7, figs. 25, 26, 27. 



External anatomy unknown. Jaw arcuate, quite strong, without 

 median projection, and absolutely smooth (pi. 9, fig. 35, P. 

 didyodes}. 



Radula composed of 22-14-1-14-22 teeth in nearly horizontal 

 series. Central tooth tricuspid, the mesocone attaining the anterior 

 border of the basal-plate, side cusps small. Lateral teeth tricuspid, 

 slightly asymmetrical. Marginal teeth also tricuspid, the entocone 

 and mesocone united at their bases (pi. 9, fig. 36, P. didyodes}. 



Genitalia : Penis stout, extending into a long flagellum (?), the 

 vas deferens inserted high upon it; the stout lower portion bearing 

 a globose appendix, at the base of which the retractor is inserted. 

 Vagina is short, muscular and swollen. Spermatheca very large 

 and long, its duct short ; (in the figure is shown a spermatophore 

 within it). Albumen gland small ; hermaphrodite duct long, not 

 convoluted (pi. 9, fig. 37, P. didyodes}. 



The notable generic features of the anatomy are that all of the 

 teeth are tricuspid (as in many Endodontas) ; the jaw is smooth, not 

 vertically striated ; the penis bears a flagellum and apparently an 

 appendix. The most important shell characters are the solidity, and 

 the callous thickening of the baso-columellar lip. 



I have considered Pararhytida a genus separate from Endodonta 

 mainly on account of the smooth jaw. In Endodonta, Pyramidida, 

 etc., the jaw is always laminate or striate. In Pararhytida its com- 

 ponent laminae seem to be completely fused. The characters of the 

 foot must be examined before we can intelligently discuss the system- 

 atic position of Pararhylida. Our knowledge of its anatomy is due 

 to Fischer (Journ. de Conchyl., 1875). 



P. dictyodes Pfr. iii, 95. P. mouensis Cr. iii, 95. 



v. dictyonina Euth. viii, 134. 



Section Saissetla (Bayle) Auc., 1889. 



Shell solid, depressed-globose or subdiscoidal, the spire slightly 

 convex ; umbilicus rather narrow. Whorls rapidly increasing, the 



