196 PEOCEEDINGS OF THE MALA.COLOGICA.L SOCIETY. 



2. SiLiQUA COST ATA (Say). 



Siliqua eostata is very similar to S. pattda, tlie chief differences 

 disceruible being : ( 1 ) the groove in which rests the clavicle or rib of 

 the shell runs a little more antero-ventrally than in S. patula, so that 

 the anterior adductor muscle of S. eostata is forced a little antero- 

 dorsally ; (2) the large pallial fringe at the middle of the ventral 

 surface is proportionately longer and narrower, and has a dendritic 

 appearance; (3) the free portion of the siphon is smaller, while the 

 exterior border between it and proximal portion of the siphon is 

 more pronounced, this being particularly so at the dorsal and ventral 

 surfaces. It may, however, be partly due to contraction. No material 

 difference is noticeable in the internal structure. 

 3. Ensis directus (Conrad). 



Ensis directus closely resembles E. ensis, but is more massive and 

 decker. Both sides of the posterior end of the proximal portion of 

 the siphon curve outwards instead of inwards. The free portions 

 of the siphon are more prominent, and carry a denser mass of ten- 

 tacular fringe. The fourth aperture is more central, that is, it is nearer 

 the anterior end. It is also longer, and bordered by several rows of 

 tentacles. 



There is no appreciable difference in the internal structure. 



4. Ensis minor, Dall. 



Ensis minor also closely resembles E. ensis, but is very much smaller 

 and moi-e attenuated. It curves more dorsally, and the fourth aperture 

 is situated at the centre of the ventral surface. 



There is no marked difference in the internal structure. 



EXPLANATION OF PLATE XII. 



Siliqua patula (Dixon). 



YiG. 1. View from the right side, showing the alimentary canal, etc. Natural size. 



2. Longitudinal section of the stomach, showing internal structure of the left 



side. X 1^. 



3. Lono-itudinal section of the stomach, showing internal structure of the right 



side. X 1^. 



4_ Transverse section of the gill, cut horizontally to the axis of the demi- 



branch. x 60. 



Reference Letters. 

 J. anus; A. A. anterior adductor muscle; A.L. anterior or upper lip; B.V. 

 blood-vessel; G. chitin in intertilamentar junction; O.C. cfecum of the crystalline 

 stvle ; C.S. crystalline style; C'.St. cardiac division of the stomach; CD. central 

 division of the stomach ; Jix.S. proximal portion of the exhalent siphonal chamber; 

 Jix.a'. free portion of the exhalent siphonal chamber; F. foot; l.J. interlamellar 

 i unction ; In. intestine; In.S. proximal portion of the iuhalent siphonal chamber ; 

 In.S'. free portion of the inhalent siphonal chamber; 7.6'. interlamellar septum; 

 Z.* liver; M. mouth; 3I.R.C. muscular ridge separating the cardiac from the 

 pyloric division of the stomach ; M.R.F. muscular ridge separating the central from 

 the pyloric division of the stomach ; Oe. ojsuphagus ; Oe. G.R. ridge dividing the 

 oesophageal from the cardiac division of the stomach ; Oe.St. oesophageal division of 

 the sttmiach; F.A. posterior adductor muscle; F.L. posterior or lower lip; 

 F.H. pallial ridge ; P.R.F. retractor pedis posterior muscle; P.St, pyloric division 

 of" the stomach ^ R. rectum ; R.F. restored portion of the foot ; S.R. ridge dividing 

 the inhalent from the exhalent chamber of the proximal portion of the siphon ; 

 St. stomach ; T.F.jU. transverse pedal muscles ; V. ventricle. 



