ISCHNOCHITON. 83 



centric growth-marks. Tail valve half-circular, its latter part 

 sculptured like the head valve ; its central area from J to more 

 than i the length of the posterior slope. Umbo acute in young 

 examples. 



The insertion plates are all overhung by the eaves, the latter pro- 

 jecting beyond them. The anterior valve has 17, posterior valve 

 12 teeth. 



The ground color of the shell has a light reddish-white or bril- 

 liant red tone, with more or less distinct radiating flamed streaks of 

 lighter and darker; girdle yellowish-brown, lighter toward the 

 edge, with few or numerous transverse tracts of lighter. (Pffr.} 



Length 2(5, breadth 14 mill. 



South Georgia. 



Trachydermon steinenii PFFR., Moll, von Siid-Georgien, in Jahrb. 

 Hamb. Wiss. Anstalten, iii, p. 103, t. 3, f. 1, 1886. 



Closely allied to T. puniceus, but different in having the central 

 and lateral areas equally granulated, and the central area entirely 

 without longitudinal sculpture. 



Section Trachyradsia Carpenter. 



Trachyradsia CPU., MS. DALL, Proc. U. S. Nat. Mus., Jan., 

 1878, p. 1, 323. Type, Ck. fulgetrum Reeve. 



Chitons like Trachydermon except that the insertion plates of the 

 median valves have two or several slits. 



The species of this group should be carefully compared with 

 Callochiton and Stereochiton, with which groups they agree in the 

 multiple slits of the central valves and the markedly spongy eaves. 



I am not disposed to group these forms with Trachydermon but 

 leave them here in deference to the views of Carpenter. It should 

 be noted that Carpenter did not consider Ch. fulgetrum the type of 

 his group, that species having been selected by Dall. 



I. FULGETRUM Reeve. PI. 8, figs. 80, 81, 82. 



Shell ovate, a little attenuated in front, very minutely and closely 

 decussately punctured under the lens, throughout. Terminal 

 valves and lateral areas of the rest somewhat obscurely radiately 

 grooved. Reddish-brown, peculiarly painted with longitudinal yel- 

 ] ow and black zigzag waves. Girdle horny. (Eve.) 



Habitat unknown. 



