6lO SAMPSON. [Vol. XI. 



and blood supj^ly, are absent, and the mantle chamber is low. 

 There is no division of fibres of this description into anterior 

 and posterior groups, but the fibres are found more or less 

 interruptedly under a large part of shell I, and posteriorly even 

 under the anterior part of shell II (Fig. 9, //). 



More anteriorly, where the shell becomes considerably nar- 

 rower, are two additional sets of dorso-ventral muscles (Fig. 3, 

 dv'a, dv"a, which, with the dorso-ventral muscles already 

 described, may be regarded as parts of one system, making a 

 condition similar to that under VIII ; under I, the system is 

 divided into three parts by two large groups of horizontal 

 muscles to the radula, that are attached to the anterior part of 

 the shell and cross between the groups of dorso-ventral fibres 

 to the buccal mass {cf. Fig. 3 and Fig. 13, I). The most ante- 

 rior dorso-ventral muscle {cf. Fig. 3, dv'a) is separated from 

 the corresponding muscle of the other side, by an oblique 

 median muscle to the anterior lip (Figs. 3, oinla, 8, oinl)\ and a 

 small horizontal muscle (Fig. 8, ]L7nl) attached more anteriorly 

 to I, runs in the median line, also to the anterior lip. 



The lips are furnished with other conspicuous muscles, 

 (i) Circular muscles (Figs. 7, 8, cl), most of which form an 

 incomplete ring around the mouth. Anteriorly where the ring 

 is not closed the fibres pass on either side of the oblique 

 median lip muscle and are there lost ; some of the circular 

 muscles, however, cross posterior to the mouth and pass hori- 

 zontally into the foot, and a few circular fibres are found in 

 the anterior lip. 



(2) Longitudinally directed fibres (Fig. 8, rp) that originate 

 in the foot and spread like fingers in the posterior lip betw.een 

 the circular fibres. 



(3) Shorter and less prominent, spreading fibres in the 

 anterior lip (Fig. 8, rd). 



Muscles of the Mantle. 



The mantle is armed on the dorsal surface with calcareous 

 spicules ; ventrally it is smooth and forms the boundary of the 

 mantle chamber, in which lie the gills, and which receives the 



