FAMILY PATELLID.E PATELLOIDA. 163 



Mr. Couthouy, to whom we are indebted for our first anatomical acquaintance with this 

 animal, observes, that " perhaps it would be more correct to consider it as a constant variety 

 (of P. testudinalis), than as a distinct species ;" and " many species have been received as 

 valid, upon far narrower distinctions than exist between this and Mr. Say's shell." It occurs 

 almost universally upon the Eel-grass ( Zostera marma), while the testudinalis is attached to 

 rocks. 



FAMILY CHITONIDM. 



Animal without tentacles or eyes, hut furnished ivith a small veil. The branchial apparatus 

 formed by a cordon of small pyramidal leaves, around the mantle. Shell multivalve, 

 shield-shaped. 



GENUS CHITON. Lmnaus. Lamarck. 



Animal elongate, obtuse at both ends, and without a very distinct head. Tentacles replaced 

 by a small membranous veil, which extends over the mouth ; the latter inferior, without 

 jaws, and with a small prickly tongue. Foot elongated, the mantle extending beyond it 

 more or less completely ; the gills under the edge of the mantle, particularly beliind. Vent 

 at the posterior extremity. Generative organs double ; one on each side, between the 

 leaves of the gills. Shell oval, composed of eight arched pieces arranged in a series more 

 or less overlapping each other, their sides imbedded in the skin. 



Chiton albus. 



PLATE X. FIG. 200. 



Chiton nlbus. MoNTiGC, Test. Brit. 4. 



C. sagrinatus, CouTHOUY, Bost. Journ. Nat. History, Vol. 2, p. 82. 



C albus. Gould, Invertebrata of Massachusetts, p, 150, fi?. 23. 



Description. Shell small : valves with a small beak, minutely crenulate on their anterior 

 margin, subcarinate with minute stria; ; the surface, under the lens, exhibiting the appearance 

 of shagreen. An obsolete diagonal ridge sometimes divides each side into triangular areas, 

 but for the most part without any distinct boundary. Margin membranous, covered with 

 beaded granules. 



Color. Epidermis a blackish powder, underneath which greyish white ; the marginal 

 membrane ash-colored, with a narrow black line in the middle surrounding it. 



Length, 0-4. Width, 0-15. 



This species was originally discovered by Mr. Couthouy in the stomachs of fishes off the 

 coast of Massachusetts, and described by him under the appropriate name of sagrinatus. It 

 has since been referred to the albus of Montagu, and aselloides of Lowe, by Dr. Gould. 



21* 



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