PROCEEDINGS OF UNITED STATES NATIONAL MUSEUM. 405 



is a single scale-like flat uncinus, bearing an elongated thickened ridge, 

 but no cusp. 



1 



Formula 



1^ + 2 + 2-2 + 2 +^)1 



This family might be incorporated with the last were it not for the 

 dift'ereuces in the branchiai and in its dentition. These latter are of 

 great weight. The dentition of Addisonia is like nothing known in the 

 whole group of Rhiphidoglossa, but, while it recalls the dentition of the 

 Chitonidic in some features, has a decidedly Docoglossate aspect. Per- 

 haps the most rational hypothesis is that this group bears to the pre- 

 ceding family much such a relation as in Pulmonata is borne by the 

 Cyelotacea of Troschel toward the Cyclostomacea. Indeed, the resem- 

 blance of the radula of Gocculina Rathhuni to that of some of the species 

 of Ilelicina figured by Troschel is quite remarkable. This family con- 

 tains, so far as known, but one genus. 



Genus Addisonia* Dall. 



Shell ovate, subcouical, strongly asymmetrical, porcellanous, thin; 

 with a blunt apex curved backward, dftwnward, and to the left, without 

 an epidermis ; with an unthickened, simple, entire margin ; pedal mus- 

 cular impression horseshoe-shaped, interrupted in front. Soft parts: 

 head provided with two tentacles without eyes or eye tubercles ; muz- 

 zle plain, simple; foot thin, orbicular, without lateral or posterior tuber- 

 cles, processes, or fringes ; mantle edge simjile, thickened; gill composed 

 of leaflets as in Patella, the series starting on the right behind the head 

 and continued within the mantle edge backward, the body of the animal 

 being asymmetrically placed with regard to the aperture of the shell to 

 afford room for the enormous series of branchial leaflets ; anus opening 

 behind ami above the head slightly to the right of the median line, and 

 indicated by a small papilla. 



Eadula: See description of the family. 



Type and only species yet known. 



Addisonia paradoxa, n. s. 



Shell ovate, thin, whitish; apex presenting an appearance as if an 

 embryonic tip (perhaps spiral) had fallen and been replaced by a pecu- 

 liarly blunt ovate apex, which in the young shell is nearly marginal pos- 

 terior and to the left of the middle line, but in the adult is considerably 

 within the margin, curved downward and backward, and much more 

 asymmetrical; sculpture of faint grooves radiating from the (smooth) 

 apex and reticulated by the stronger concentric lines of growth, beside 

 which the extremely inflated arch of the back is somewhat obscurely 



* la honor of Prof. Addison E. Verrill, of Yale College and the United States Fish 

 Commission, whose surname has already been applied to more than one group of in- 

 vertebrates. 



