TYLODINA. 185 



Hyalopatina riishii DALL, Blake Gastrop., p. 61 (1889). 



This remarkable shell has been some time in the National Mu- 

 seum, and has been submitted to several conchologists, and studied 

 with much care. In the absence of any further information, I have 

 come to the conclusion that it may be related to Umbraculum, from 

 which, conchologically, it is separated by its oval form, posterior 

 nucleus, and granulated surface. The discovery of a living speci- 

 men, however, may show the true relations of the creature to be 

 elsewhere. It has a little the general appearance of an extremelv 

 thin, flat Crepidula ungniformis without a deck, and with the nucleus 

 within the margin (Z)a//). 



The figure is from a drawing kindly lent by Dall. 



Genus TYLODINA Rafinesque, 1819. 



Tylodina RAF., Analyse de la Nature, ou Tabl. de I'lJiiivers et 

 des Corps Organises, 1815, p. 143 (nude name) ; Journal de Physique, 

 de Chimie, d'Hist. Nat., etc., Ixxxix, p. 152 (1819). Joannisia 

 MONTEROSATO, Nomencl. Gen. e Spec. Conch. Medit., p. 149 (1884), 

 type T. citrina. 



Animal almost completely retractile under the shell ; foot flat, 

 broadly truncate in front, pointed behind ; head distinct, elongated 

 and sub-bifid in front, with a buccal veil terminating in labial ten- 

 tacles, the mouth between buccal veil and foot ; dorsal tentacles 

 long, cylindrical, slit on the outer sides almost their entire length, 

 with olfactory lamellae within the slit. Eyes sessile at their interno- 

 anterior bases ; mantle completely covered by shell, its edge den- 

 ticulate. Branchial plurne bipinnate, free for the greater part of its 

 length, lying between mantle and foot on the right side, the end 

 projecting backward. Anus behind the gill ; genital pore in front 

 of gill. 



Radula armed with many similar narrow teeth ; jaws rudimen. 

 tary ; gizzard with numerous corneous denticles. 



Shell external, conical and limpet-like, calcareous except at the 

 borders which are membranous ; apex recurved, when perfect com- 

 posed of two glossy, vitreous whorls, coiled spirally to the left and 

 backward. 



Distribution : Mediterranean, Gulf of Mexico, California. 



This genus differs notably from Umbraculum in having the shell 

 larger in proportion to the body, in the projecting head, not included 

 in an anterior sinus of the foot, and in the external genitalia. 



