35 



Figure. 



Scutella crenulata. PI. 24. Fig. 140 and 141. Shell, showing its ex- 

 ternal cancellated sculpture and internal muscular impression. 



Family 2. CHITONACEA. 



Shell ; oblong-dish-shaped, of eight separate pieces, either set in a 

 cartilaginous frame, or in the mantle separately. 



Of all mollusks the Chitons are they which naturalists have been most 

 puzzled to know what to do with. Milne-Edwards doubted if they were 

 Mollusca at all; and De Blainville showed that he was much of the same 

 opinion by classing them, like Linnaeus, with the Barnacles. Cuvier 

 brought them into relationship with the Patella, after the manner of 

 Adanson, on account of the similarity of the breathing-system, and na- 

 turalists are now pretty well agreed upon this arrangement. The genera 

 are — 



Chiton. Chitonellus. 



Genus 1. CHITON, Linnceus. 



Animal ; head obscurely defined, formed of a proboscis covered by a 

 fleshy semicircular hood ; no tentacles nor eyes; branchia form- 

 ing a series of lamellae between the mantle and foot on each 

 side ; foot an oblong dish, rounded at each extremity. 



Shell ; oblong or elongately ovate, eight-valoed, valves moving one 

 upon another, convex, umbonated in the middle, someiohat 

 ivinged on each side, fixed in a cartilaginous frame, which is 

 either smooth, squamate, hairy, spinous, or fasciculate, some- 

 times ividely expanded, sometimes covering the valves entirely. 



Unlike Patella, the animal of Chiton is a blind oblong mass, without 

 tentacles, and with scarcely a definable head, yet the branchiee are similar, 

 and the shell may be likened ideally to a Limpet divided into eight pieces. 

 Dr. Gray considers the posterior terminal valve to be the homologue of 

 the shell of Patella. The eight separate pieces, or valves, are sustained 

 in their place by a cartilaginous frame, a horny extension of the mantle, 

 and they move one upon another, like plate-armour, by the aid of three 

 flexible muscles, attaching crosswise to each valve. This horny frame or 

 ligament is distinguished by the following varieties of character. In 



