4U TONICELLA. 



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

 eaves conspicuous ; -inns narrow, deep, smooth. Girdle leathery, 

 minuU'ly dowpy-ecaled and having occasional delicate, corneous 

 hair*. Opr.) Length 25, breadth 11 1 mill.; divergence 100. 



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



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

 with a triangular spot. (Q/r.) 



Genus VIII. TONICELLA Carpenter, 1873. 



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

 reus Fabr. DAI.I, IVoc. U.S. Nat. Mus. 1878, p. 324. Tonin.i, 

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

 Reg. Arct. N..rv. p. 116, 1878. 



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

 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 irrouj) differs from Trachydermon, Callochiton, Ischnochiton, 

 Ckitniileurn, etc., in its naked, scale-less girdle ; from Stereochit.m 

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

 Tn iria, 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 Chtptopleura seem 

 to ! bridled 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- 

 pro posed for the reception of the C. hartwegiiCpr. The genus 

 will therefore be divided thus: 



Suhgenus TONMT:I.I.A s. s. 

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



Subgenus CYANOPLAX Pils. 



(h stumpy, bi- or tri-loh<-d ; valves granulated; gills ambient. 



