40 TONICELLA. 



Posterior valve with 9, anterior 9, central 1 slit. Teeth acute ; 

 eaves conspicuous ; sinus narrow, deep, smooth. Girdle leathery, 

 very minutely downy-scaled and having occasional delicate, corneous 

 hairs. (Cpr.) Length 25, breadth 11} mill.; divergence 100. 



Is. Gorriti (Mus. Cum. No. 34). 



One of the specimens is light colored on the jugum of each valve, 

 ancircled with a triangular spot. (Q?r.) 



Genus VIII. TONICELLA Carpenter, 1873. 



Tonicella CPR., Bull. Essex Inst. v, p. 154, 1873, type C. marmo- 

 reas Fabr. DALL, Proc. U. S. Nat. Mus. 1878, p. 324. Tonicia, 

 in part of GRAY, ADAMS, et at. Boreochiton (part) SARS, Moll. 

 Reg. Arct. Norv. p. 116, 1878. 



Valves, sutural plates sinus and teeth as in Ischnochiton, but the 

 eaves are spongy ; girdle as in Tonicia, leathery, smooth or nearly 

 so. Gills extending forward from two-thirds to three-fourths the 

 length of the foot. 



Distribution, Northern Atlantic and Northern Pacific shores. 



This group differs from Trackydermon, Callochiton, Ischnochiton, 

 Chcetopleura, etc., in its naked, scale-less girdle ; from Stereochiton 

 in having a single slit in the median insertion plates. It differs from 

 Tonicia, to which genus the species of Tonicella were formerly referred, 

 in the non-pectinated insertion plates, spongy eaves and shorter gills. 



The surface of the valves is smooth, or evenly microscopically 

 granulated. 



The essential differences between Tonicella and Chcetopleura seem 

 to be bridged by several species which are intermediate in charac- 

 ters of gills, girdle or both. The former should perhaps rank as a 

 subgenus rather than a genus, as the pattern of sculpturing of the 

 valves, shorter gills and spongy eaves are about all there is to sep- 

 arate the two groups. 



Under Tonicella, as a subgenus, I include the peculiar type Cyano- 

 plax, proposed for the reception of the C. hartwegiiCpr. The genus 

 will therefore be divided thus : 



Subgenus TONICELLA s. s. 



Teeth rather long; valves nearly smooth ; gills shorter than the 

 foot. 



Subgenus CYANOPLAX Pils. 

 Teeth stumpy, bi- or tri-lobed ; valves granulated ; gills ambient. 



