654 DESCRIPTION OF PLATES, 20, 21. 



Fig. 4 c, Catophragaiua polyraerus, outline of the basal edges of all 



the valves of a shell ; a, ros- 

 trum ; b, rostro-lateral ; c, 

 lateral ; d, carino-lateral 

 compartments ; e, carina. 

 4 d, ,, ,, external view of a portion of the 



shell of a much corroded and 

 worn down specimen, serving 

 almost as a transverse section , 

 c, lateral compartment, d, 

 carino-lateral compartment. 

 4 e, ,, ,, scutum and tergum, internal 



views of. 



PLATE 21. 

 Genus — Ve rrtj c a . 



N. b. — Letters of reference the same throughout the Plate. 



a, rostrum. 



B, carina. 



s, moveable scutum, s', scutum fixed and modified so as to form 



part of the shell. 

 T, moveable tergum, t', tergum fixed and modified so as to form 



part of the shell. 

 In s, and s', a is the occludent margin ; b, the basal margin ; 

 m, the plate to which the adductor muscle is fixed ; o, a ledge 

 formed during continued growth, in the upper part of the 

 fixed scutum, in order to keep the orifice perfectly closed ; 

 this ledge is seen distinctly only in fig. 5. 

 In 9 and s' the tergal margin is marked by small dashes; (') being 

 the upper articular ridge, and ("), the second or lower articular 

 ridge : in s' (") is called the parietal portion of the valve. 

 In t and t', the scutal margin is marked by small dashes ; (') being 

 the first and upper articular ridge, hardly distinct from the 

 occludent margin, and called in t" the occludent rim ; (") is the 

 second or middle, and ("') the lower or third articular ridge, 

 called in t" the parietal portion of the valve : #is the carinal 

 margin, called in t' the carinal rim, and z the basal margin ; 

 o is the occludent margin : in t', a ledge is formed across the 

 under side close to the apex, which ledge is necessary to keep 

 the orifice neatly closed, owing to the apex of the moveable 

 tergum coming to project freely during growth. 

 Fig. 1 a, Verruca Stromia, specimen with the left hand scutum and 

 tergum fixed and modified into part of the shell. 

 1 b s Verruca Stromia (with left hand scutum and tergum fixed), 

 with the valves and compartments separated from each 

 other ; the homologous parts of the opercular valves arc 

 marked by corresponding letters and dashes. 

 1 c, Verruca Stromia, left hand scutum and tergum fixed, viewed 

 from the under side. 



