204 Dall A New Chiton from the New England Coast. 



three) radial riblets with a tendency to bifurcate or break up into segments 

 distally ; there are no distinctly marked areas on the midvalves, but the 

 part of the valves which bears what in many chitons are called the lateral 

 areas, in this species carries two or three, sometimes bifurcate, thread-like 

 ribs which are flattened above and rarely reach the mucro of the valve, 

 being usually evanescent dorsally ; there are also more or less deeply im- 

 presse'd lines of growth ; the median part of the valves is nearly smooth 

 except for the microscopic punctation ; anterior valve semicircular, with 

 numerous, more or less irregular, radial riblets that resemble those on the 

 midvalves ; the eaves are conspicuously spongy ; the insertion plates are 

 blunt, crenulate at the edge, but not radially striate ; the anterior valve in 

 the type has 10, but would seem normally to have either 9 or 11 slits, as one 

 seems missing or in excess, on one side; the midvalves have one slit on 

 each side, their anterior lamellae are nearly continuous across the dorsal 

 sinus; the posterior valve is small without a mucro, the incremental lines 

 strong, the ribbing obsolete or nearly so ; in the type, while there is not a 

 posterior sinus like that of Chsetopleura , there is a certain flattening and the 

 insertion plates bordering this part of the valve are poorly developed ; 

 there are 7-9 slits between which the distal edges of the plates are more 

 or less irregularly crenate; in the dried specimen the muzzle has a promi 

 nent " veil " or tegumentary margin ; the ctenidia number about 15 on each 

 side and extend forward on each side of the foot to the fourth valve ; 

 length over all (dry), 13 mm.; width, 8 mm.; dorsal angle, 120. 



Dredged in 20 fathoms off Ironbound Island, Frenchman's Bay, nearMt. 

 Desert Island, Maine. 



From Trachydermon ruber, Tonicella marmorea and similar species, this 

 form can be at once distinguished by the ribbing. If the type specimen 

 be characteristic in its color, the pattern and hue would be equally dis 

 tinctive. In T. ruber the girdle is pubescent and particolored, in T. mar 

 morea and the present species it is of a uniform brown. The type specimen 

 has been generously donated to the U. S. National Museum. 



