THE NAUTILUS. 31 



NOTES ON THE ACANTHOCHITID^ WITH DESCRIPTIONS OF NEW 

 AMERICAN SPECIES. 



BY HENRY A. PILSBRY. 



The family Acanthochitidse 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 Cryptoplacidce also share this peculiar plan 

 of valve-sculpture, but they are vermiform in shape and not nearly 

 covered above by the valves, whilst the Aeanthochitidce have well- 

 developed valves covering the upper surface, even in those genera 

 like Amicula and Cryptochiton which have the girdle-skiu extend- 

 ing over the larger part or the whole of the dorsal armor. There 

 are many other differences, but still the Cryptoplacidce give unmis- 

 takable evidence of their descent from Aeanthochitidce. On the 

 other hand, all other Chitons difier 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 Acanthochitidse : Sjwngiochiton, 

 Leptopla.v, Acanthochites, Katharina, Amicida, Cryptochiton. All 

 but the first two are found upon the United States coasts. It will 

 be noticed that the association of Acanthochites with 3Iopalia, 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 

 steUeri. 



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 



