PLATE IV 



i, A to D. Diagrammatic representation of four stages in the growth 

 of a Gastropod, to show the process of torsion of the body (after 

 Boutan) : 



A, Embryo, with alimentary canal straight. 



B, Later stage, with posterior portion of the alimentary canal bent 



beneath the body. 



C, Further stage, showing the developing shell curving forwards 



over the animal's head. 



D, Final stage, in which the body has twisted in the direction- 



shown by the arrow, bringing the coil of the shell to the rear 

 and the posterior termination of the alimentary canal to the 

 right front : /. , foot ; m, , mouth ; s. , shell ; p. c. , pallial cavity ; 

 v., velum. 



2, a, b. Two views of the apex of Ttcrbonilla dipsycha, Watson. From 

 an enlarged model in the Natural History Museum. 



3, A to C, B',C. Diagrammatic illustration of the transition from a 

 flat, coiled shell : A, through B and C to right-handed spiral (dextral), 

 or through B' , C to a left-handed spiral (sinistral), irrespective of the 

 anatomy of the animal (after Lang). 



4, A to F. Diagrams illustrating the different forms of mantle open- 

 ings and the formation of siphons in bivalve shells (after Lang.) : 



A, Mantle open all round. 



B, Mantle margins applied to each other except at e. (exhalent 



aperture) and i. (inhalent aperture), and where /. (the foot) 

 protrudes. 



C, Mantle margins grown together at I. 



D, Mantle margins grown together at I. and II. 



E, Mantle margins prolonged into siphons, e. and i. (as before), 



united at I. and II. 



F, Siphons united, L, also mantle margins at II. and III. 



5, A to E. Diagrams illustrating the different forms of Pelecypod gills 

 (after Lang) : 



A, Protobranch ; B, Filibranch ; C, Eulamellibranch ; D, Septi- 

 branch ; E, enlargement of ciliated junctions. 



* * All drawn by Miss G. M. Woodward. 



