POLYPLACOPHORA. XI 



the girdle than is possessed by Lepidopleurus ; and in fact we find 

 that its insertion plates are reduced to very short stumps. The 

 anterior " teeth " are small, low, unevenly notched, and not nearly 

 as long as the overhanging eaves. It is only necessary to point out 

 that the genus Plaxiphora, the nearest ally of Placophoropsis, is 

 remarkable for the length of its strong, regularly slit insertion 

 plates. The same is true of the entire family Mopaliidce, to which 

 these genera belong. In Placophoropsis atlantiea we have, there- 

 fore, a member of a family of littoral Chitons, which by descent into 

 the abyssal region has partially lost by retrogressive development 

 through disuse the characters originally evolved to fit the ancestral 

 forms for existence in the littoral zone. Further deep sea researches 

 will probably reveal other cases of degeneration along the lines here 

 indicated ; and it is not impossible that forms as destitute of insertion 

 plates as the genus Lepidopleurus have been produced from higher 

 types by retrogessive evolution ; but there is at present no evidence 

 of the existence of such forms. 



THE GIRDLE. 



This organ variously called ligament, zone, or girdle, is a band 

 of connective and muscular tissues extending around the valves, the 

 edges of which are imbedded in it. The surface of the girdle is 

 variously clothed with scales, spines, haiis or spicules, or is naked. 

 The armor, whether of spines, scales or hairs, consists of a chiti- 

 nous foundation more or less infiltrated with lime. The girdle- 

 appendages vary greatly in development among closely allied 

 species ; but the general type of covering is a moderately constant 

 generic character. In some forms the girdle encroaches upon or 

 entirely covers the valves. In some genera having a posterior 

 sinus in the tail-valve, the girdle has a corresponding sinus or slit ; 

 in others it has no sinus, but in life is always curved upward into a 

 posterior conduit, serving for an outlet for the anus and gills. 



In some forms having hairy or spiculose girdles, the hairs or 

 spicules are gathered into tufts or groups at the sutures (i. e. lateral 

 points of junction between valves), and around the anterior valve; 

 issuing from pits or pores at those places (see pi. 5, fig. 97). In 

 their incipient stage, these sutural tufts are merely the gathering 

 together of two or several of the ordinary surface hairs ; in their 

 full development they are dense clusters of long spicules ; and in 

 their decadence only a small pit or pore remains. The presence of 



