THE NAUTILUS. 89 



the valves outside, in the type specimen, is fairly uniform but prob- 

 ably more or less variable among individuals ; the valves inside are 

 of a deep rose-pink, paler toward the edges ; surface minutely more 

 or less quincuncially punctate, but this is visible only when magni- 

 fied ; the sculpture consists of (on the mid valves usually three) radial 

 riblets with a tendency to bifurcate or break up into segments dis- 

 tally ; 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 impressed lines of growth ; the median part of the 

 valves is nearly smooth except for the microscopic punctation ; an- 

 terior 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 in- 

 cremental lines strong, the ribbing obsolete or nearly so ; in the type, 

 while there is not a posterior sinus like that of Chcetopleura, 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 tin- 

 dried specimen the muzzle has a prominent '' veil " or tegumentary 

 margin ; the ctenidia number about 15 on each side and extend for- 

 ward on each side of the foot to the fourth valve ; length over all 

 (dry), 13mm.; width, 8 mm.; dorsal angle, 120°. 



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

 near Mt. 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 lie characteristic in its color, the pattern and hut- 

 would be equally distinctive. In T. ruber the girdle is pubescent 

 and parti-colored ; in T. marmorea and the present species it is of a 

 uniform brown. The type specimen has been generously donated to 

 the U. S. National Museum. 



