THE NAUTILUS. 31 
NOTES ON THE ACANTHOCHITIDZ WITH DESCRIPTIONS OF NEW 
AMERICAN SPECIES. 
BY HENRY A. PILSBRY. 
The family Acanthochitidz includes Chitons having the exposed 
surface of the valves, when present, divided into a narrow dorsal 
smooth or striated band, sometimes obsolete, with a granular area on 
each side, formed by the union of the lateral areas and the plural tracts 
of the central areas. The Cryptoplacide also share this peculiar plan 
of valve-sculpture, but they are vermiform in shape and not nearly 
covered above by the valves, whilst the Acanthochitide have well- 
developed valves covering the upper surface, even in those genera 
like Amicula and Cryptochiton which have the girdle-skin extend- 
ing over the larger part or the whole of the dorsal armor. There 
are many other differences, but still the Cryptoplacide give unmis- 
takable evidence of their descent from Acanthochitide. On the 
other hand, all other Chitons differ in having the valves divided into 
triangular lateral, and wide central areas, and in other equally 
important if less obvious features. 
The following genera belong to Acanthochitide : Spongiochiton, 
Leptoplax, Acanthochites, Katharina, Amicula, Cryptochiton. All 
but the first two are found upon the United States coasts. It will 
be noticed that the association of Acanthochites with Mopalia, insti- 
tuted by Dr. Philip Carpenter, is not retained. 
Some naturalists may find it difficult to believe that complex 
structures so very similar to each other as are the posterior valves in 
Mopalia and Acanthochites could have arisen independently ; but 
that this is the fact I feel entirely assured. In the two cases, this 
peculiar form of two-slit and sinused posterior insertion-plate, arose 
from a perfectly regular, even, and many-slit plate; the two phyla 
travelling along parallel roads. The Mopaloids reach their culmi- 
nation in Plaxiphora, which has lost its two posterior slits, and is in 
this respect quite analogous to an old individual of Cryptochiton 
stelleri. 
The genus Acanthochites, which has given its name to the family, 
is readily recognized by the series of tufts of fine bristles, like spun- 
glass, along each side. These tufts may be accounted for by the 
theory that they are the result of over-nutrition caused by the fre- 
quent flexure of the girdle at the sutures; this flexure naturally 
bringing a greater share of nutriment to the stimulated point than 
