UNIVERSITY Ot- ^ 

 NATURAL SCIENCES OP PHILADELPHIA. 199 



In Mesodon, Triodopsis^ and Arionta, the marginals are longer 

 than wide, with only two, sometimes bifid cutting points. In 

 Stenotrema and Polygyra they are rather wider than long, also 

 with two more bluntly bifid cutting points. It must be borne in 

 mind, however, that my observations have not led me to believe 

 these characters sufficiently constant to be of subgeneric value. 

 I prefer to wait till more species have been examined. There is 

 also some variation in the mode of passing from lateral to mar- 

 ginal teeth, even in the same subgenus. These points will be 

 treated more fully under each subgenus. 



Subgenus Gonostoma. 



This subgenus is represented in our limits by one species onl}'^, 

 H. Yatesi, J. G. Cooper, not Pfr., whose jaw and lingual membrane 

 are here described. Jaw (pi. XVI., fig. 10) low, wide, slightly 

 arcuate, ends scarcely attenuated, blunt; cutting margin without 

 median projection ; anterior surface with a strong transverse line 

 of reinforcement, and numerous, about 12, wide, crowded ribs den- 

 ticulating either margin. 



Lingual membrane (pi. IX., fig. 3) long and narrow; teeth 24 — 

 1 — 24, with 6 perfect laterals. Centrals with the base of attach- 

 ment longer than wide, with expanding lower lateral angles, and 

 squarely reflected upper margin ; reflection large, stout, bearing 

 small but distinct side cusps, with short, blunt cutting points, 

 and a long, stout, median cusp reaching the lower edge of the 

 base of attachment, beyond which projects the long, acute cutting 

 point. Laterals like the centrals, but unsymmetrical by the sup- 

 pression of the inner, lower, lateral angle of the base of attach- 

 ment, and the distinct inner side cusp and cutting point. Mar- 

 ginals subquadrate (6), a simple modification of the laterals, the 

 reflection being more developed, and bearing one inner, oblique, 

 long, blunt cutting point, and one smaller side cutting point; the 

 extreme marginals (c) are rather wider than high, and the cutting 

 points are bluntly rounded. 



The name Yatesi being already preoccupied in the genus Helix, 

 Dr. Cooper's species may be known as Yatesiana. 



Von Martens describes the jaw of Gonostoma as having distinct 

 ribs. Moquin-Tandon so figures that of obvohda, Miill, lenticula, 

 Fer, and Rangiana, Fer; and Gassies (Journ. de Conch., XV., 

 1861, 15) so describes that oi H. constricta, B. 



