ACANTHINULA. 



341 



Fig. 223. 



15 perfect laterals; the change from laterals to marginals is made without 

 the splitting of the inner cutting point The 

 centrals and first laterals have no distinct side 

 cusps and cutting points. 



Genital system (see Leidy, 1. c.) very remark- 

 able for the enormous development of the penis 

 sac ; it is stout, cylindrical, as long as the whole 

 genital system, receiving both retractor muscle 

 and vas deferens at its summit ; genital bladder large, elongate ovate, on a very 

 short duct. 



Jaw of M. Sayii (Morse). 



ACANTHINULA, Beck. 



Animal heliciform ; mantle posterior ; other characters as in Patula (see be- 

 low, Fig. 226). 



Shell perforated, globosely turbinated, with a brownish plicately ribbed or 

 aculeate epidermis ; whorls 4 - 5 ; aperture rounded ; peristome thin, some- 

 what expanded, its terminations approached. 



In Europe this genus is found at the north, but one species ranges as far 

 south as Palermo. Our single species is probably circumpolar, common to the 

 three continents. 



We have but one species within our limits, A. harpa, whose jaw and lingual 

 dentition have been described and figured by Morse. Judging from his figure 

 (Fig. 224) and text, the anterior surface of the jaw seems to 

 have subobsolete ribs which mark the lower margin ; it is 

 low, wide, strongly arched, with blunt, scarcely attenuated 

 ends ; cutting edge with a wide and ,very slightly produced, 

 broad median projection ; transversely and longitudinally 

 striate. 

 Lingual membrane long and narrow, 120 rows of 17 — 1 — 17 teeth, with 6 



Fig. 224. 



Jaw of A. harpa 

 (Morse). 



Fig. 226. 



Lingual dentition of A. harpa (Morse). 



perfect laterals. The centrals have a square base of attachment, the upper 

 margin squarely reflected ; the reflection is very small, tricuspid, the side cusps 

 very small, blunt, the median cusps very long and narrow, not reaching the 

 lower edge of the base of attachment, not even with its short cutting point ; 



