Mril^Ajc^iHsamcotik'oiiihe'Amtnal 0/ Myochama anomloideSi 289 



on each, only fourteen or fifteen large, transverse plates doping^ 

 from the external margin to the point : the border of this mar- ' 

 gin is smooth. ■ 1 



There is on either side of the visceral mass a single gill-plate, 

 and a rudimentary gill-plate, which, from their arrangement, 

 have much the appearance of forming but one leaflet. This 

 apparent single leaflet is of a triangular form with one of the 

 angles directed backwards, and is attached the whole extent of 

 the dorsal margin, the anterior or ventral border alone being 

 free. This border is grooved and terminates above, at the side 

 of the mouth, between the upper and lower palp ; the posterior 

 angle is united below the visceral mass to that of the leaflet of 

 the opposite side. The external suiface of the leaflet is divided 

 longitudinally by a line (fig. 1 0) into two portions — an anterior 

 or ventral, and a posterior or dorsal ; the anterior or ventral {m) 

 is much the larger, and is a perfect gill-plate, being composed 

 in the usual way of two laminae, with the space between them 

 divided into wide, transverse tubes, which open into a dorsal 

 channel. There are two such channels, one on each side of the 

 visceral mass corresponding to the gill-plate of either side, which 

 open into a great central channel, — the anal chamber leading to 

 the exhalant siphonal tube. The gill-plate is suspended from 

 the dorsal margin of the branchial chamber by a membrane, 

 which, passing under the posterior or dorsal portion of the leaflet, 

 is united to the outer lamina, the union being marked by the 

 external longitudinal line. 



The posterior or dorsal poi'tion (n) of the leaflet is formed of 

 only a single lamina, and must therefore be looked upon as a 

 rudimentary gill-plate. It is attached by the whole length of 

 its ventral margin, which is defined by the external line (0) 

 already alluded to, to the outer lamina of the gill-plate, and by 

 its dorsal border to the dorsal margin of the branchial chamber. 

 Thus an additional channel is formed beneath each rudimentary 

 gill, and external to the membrane which suspends the gill- 

 plate ; these two channels open likewise into the anal chamber. 

 There are consequently four channels leading from the breathing 

 apparatus towards the exhalant siphonal tube ; two being from 

 the origin of the perfect gill-plates, two from below the rudi- 

 mentary gill-plates. 



This ai'rangement of the branchial organ is precisely similar 

 to that of Cochlodesma, only in the latter the gill is more elon- 

 gated, consequently the channels in connexion with it are greatly 

 increased in length, and the central channel or exhalant chamber 

 becomes more obvious. From Professor Owen's description of 

 these parts in Pholadonuja, they would also appear to be arranged 



