THE ANATOMY OF TLEUROTOMARIA BEYRICHII. 263 



If these conclasions regarding the homologj'' of the kidney 

 of the Monotocardia have any truth iu them, then it would 

 be quite possible to derive the Monotocardia from a Dioto- 

 cardian having the type of kidney seen in Pleurotomaria, 

 Trochus, or Haliotis. 



As I have already pointed out, the nervous system of 

 Pleurotomaria would serve as an excellent starting-point 

 from which to derive that characteristic of the Ttenioglossa, 

 better by far than that of Trochus, which iu the chai'acter 

 of its gills more nearly approaches the Monotocardia. The 

 general lowly character of Pleurotomaria, especially of its 

 nervous system and radula, and slight reduction of the right 

 gill, taken together with its great antiquity, justifies us, I 

 think, in regarding it as a very primitive form, and one from 

 which the great monotocardian group may very possibly 

 have arisen, and possibly also some of the subdivisions of the 

 Diotocardia. 



The following is a brief summary of some of the conclu- 

 sions at which I have arrived. 



Summary. 



1. Pleurotomaria is a typical example of a zygobran- 

 chiate Diotocardian. 



2. In the absence of sharply marked specialised regions in 

 the radula Pleurotomaria Beyrichii and P. Quoyana 

 are distinctly primitive among the Rhipidoglossa. 



3. In the reduction of the right gill Pleurotomaria tends 

 to approach the azygobranchiate Diotocardia. 



4. In the uniform distribution of the o-ang-jionic cells 

 through the connectives, the commissure, and even the large 

 nerves, and the consequent absence of distinct ganglia, 

 Pleurotomaria is extremely primitive. 



5. In the position of the point of origin of the visceral 

 loop (roughly halfway between the cerebral and pedal 

 regions) Pleurotomaria approaches the archi-t^nioglossate 

 Paludina and Nassopsis. 



6. The pleural ganglion probably arises at the point of 



