186 TURBO ASTRALIUM. 



Sarmaticus. (T. sarmaticus, pi. 61, fig. 7.) This form is peculiar 

 in the central teeth, which are composed of three lamellae overlying 

 each other; the cusp is wide and simple. Laterals with cusps, and 

 basal plates projecting beyond the cusps. Marginals as in Senectus. 



Prisogaster. (T. niger, pi. 61, fig. 9.) Median tooth with a very 

 wide body, and supporting wings, cusp narrowly reflected along 

 the whole upper margin of the tooth; laterals with prominent 

 cusps, their bases denticulate; inner marginals not enlarged as in 

 the preceding groups, but rather narrow, with long simple cusps; 

 outer marginals with long serrate cusps. In the denticulate cusps 

 of the lateral teeth, and the narrow inner marginals this form ap- 

 proaches more closely than any other to Leptothyra. 



Genus Astralium. 

 A. Central teeth with cusps. 



Astralium, s. sir. (A. spinulosum, pi. 61, fig. 12.) The median 

 tooth has a long basal plate, a, upon which an accessory obovate 

 plate, b rests; upon this is placed the body, d, with ''stu'tzlamelle," 

 c, and cusp, e. The laterals are provided with cusps, and, except 

 the outer two, with supporting wings. 



Lithopoma. (A. tuber, pi. 61, fig. 8.) Median teeth as in A. 

 americanum, but with the basal plate projecting below. Laterals 

 similar, asymmetrical. In A. ccelatum the central teeth are nearly 

 as wide at the apex as at the base; cusp equally wide; otherwise 

 similar in essential characters. In A. americanum (pi. 60, fig. 71,) 

 the basal plate does not project above the cusp; the tooth is sub- 

 triangular, w 7 ith supporting wings on the sides. 



Imperator. The radula is known only by a figure published by 

 Hogg many years ago. The central and lateral teeth bear cusps. 

 The marginals are not especially large toward the inside. Xo 

 further characters can be made out from the figure. 



Guildfordia. Radula unknown. 



B. Central teeth without cusps. 



Bolma. Troschel has figured the radula of A. rugosa. My own 

 preparations show a decidedly different form from his figures ; the 

 centrals (pi. 61, fig. 13) have a narrow long basal plate, w r hich is 

 produced above the body of the tooth ; the latter is wide, oval, not 

 reflected above ; its lower margin is not well-defined in my specimens 

 which are, however, not stained ; the laterals are of the usual form 

 and bear cusps. This group does not seem to be 'more related to 



