306 PLACIPHOKELLA. 



Valves very broad and short, the middle ones much broader than 

 those toward the ends ; head valve narrowly crescentic, tail valve 

 still smaller, with posterior mucro. Sinus small ; insertion-plates 

 short and thick, the teeth lobed or rugose. Slits 8 or more in the 

 anterior, 1 in the median, 2 in the posterior valve, which has also a 

 shallow posterior sinus. Eaves spongy. Girdle widest, often very 

 wide, in front, bearing sparsely scattered, scaled hairs. 



This genus differs from Mopalia in the rotund contour of the 

 valves taken together, and their extreme shortness individually, as 

 if the shell had been crowded together from the ends. The slits are 

 practically as in Mopalia, being normally (or at least originally) 8 

 in the head valve ; but this number is frequently increased by the 

 splitting of some teeth. The mantle-edge is produced in front and 

 fringed with long fleshy finger-like processes. The larger hairs of 

 the girdle are extremely peculiar, being covered with imbricating 

 scales like a snake skin, on a corneous core. Nothing of the sort 

 has been found in any other group of Chitons. 



Carpenter originally intended this genus to include forms having 

 hair-pores at the sutures, regardless of the other characters of valves 

 and girdle ; but the presence of pores is not, in Placiphorella and 

 Mopalia, even a constant specific character. 



The species are separated by rather slight characters and owing 

 to the rarity of specimens in collections, we are not yet prepared to 

 say to what extent the characters will hold good. 



P. YELATA Carpenter, n. sp. PI. 66, figs. 6-12. 



Shell roundly oval, broad, rather depressed, quite obtusely angled. 

 Surface lusterless. Light colored along the middle, mainly oliva- 

 ceous on the sides, especially the lateral areas ; the central areas 

 variously streaked longitudinally with buff, blue, chestnut and 

 pink. 



Median valves not beaked, marked by growth lines ; the lateral 

 areas somewhat raised, having a rounded wide diagonal rib and 

 another at the sutural margin, the space between them more or less 

 excavated. Central areas having a " false beak " or narrow pro- 

 jection forward at the dorsal ridge (only visible when the valves are 

 separated.) Anterior valve crescent shaped, sculptured with light 

 concentric growth-lines only. Posterior valve small, slightly 

 waved inward behind, the mucro far backward, recurved and elevated, 

 the slope in front of it concave in profile, unless the mucro is eroded. 



