HEMIARTHRUM. 19 



p. 667. Hanleyia mendicaria CPR., N. Eng. Chitons, Ann. Mag. 

 N. H. (4) xiii, p. 121 ; MS., p. 9, 10. BALL, Proc. U. S. Nat. Mus. 

 1878, p. 319. 



H. TROPICALIS Ball. PL 6, figs. 12-18. 



Chiton about lO'O mm. in length and 5'0 mm. in width, with the 

 usual characters of the genus, i. e., anterior valve with an unslit 

 insertion plate, other valves without even the plates ; back with an 

 angle of about 90. Girdle (fig. 13) in the dried specimens, thin, 

 narrow, covered with close-set, white, glassy, slender spines (TO-1'5 

 mm. in length) above, and below with similar but much shorter 

 ones. Anterior valve with a well-marked mucro, from which 

 proceed concentric striae of growth, and in front of which the valve 

 is a little concave ; sculpture of little less tesselated flattened nodules 

 radiating from the mucro, above which are irregularly distributed 

 small, opaque, white pustular bubbles of shelly matter looking like 

 attached grains of coral sand ; posterior valve smaller, flatter, with 

 similar sculpture, but the pustules take on a decidedly cylindrical 

 shape and are elongated, looking like the stalks of decapitated 

 mushrooms; this arrangement is still more marked on the posterior 

 lateral areas of the intermediate valves, the anterior lateral areas 

 also exhibit (less prominently), but with an obliquely anteropos- 

 terior radiation, while the former have it more entirely lateral from 

 the mucrones; dorsal areas with the tesselated sculpture alone, 

 arranged longitudinally. Color entirely white. 



Sand Key, Florida, in 128 fms. 



Hanleyia tropicalis DALL, Bull. Mus. Comp. Zool. ix, p. 53, 1881 ; 

 'Blake' Gastrop. p. 415, t. 26, f. 8c, 8d ; Bull. U. S. Nat. Mus. No. 

 37, p. 172, t. 26, f. 8c, 8d. 



Genus III. HEMIARTHRUM Carpenter, 1876. 



Hemiarthrum CPR., in Ball's Moll, of Kerguelen I., Bull. U. S. 

 Nat. Mus. ii, 1876, p. 44. 



Head and tail valves having smooth, unslit insertion plates, the 

 intermediate plates lacking them. Eaves minute. Sinus broad and 

 spongy. Girdle solid, downy, poriferous, the lateral tufts small. 

 Gills short. 



Differs from all the preceding genera in having both head and 

 tail valves provided with insertion plates, and in the poriferous 

 girdle. 



