412 Messrs. M. Caullery and F. Mesnil on Spirorbis. 



the convex side and n+p rows on the concave side, where 

 ^ = 2—4. 



6. We find a series of species, both right- and left-handed, 

 in which the thorax presents, on the concave side only^ a third 

 row of nncini, representing a fourth thoracic setigerous rinj, 

 otherwise absent. In Sp. cancellatus, Fabr., this ring carries 

 besides on the concave side dorsal bristles. 



This group of arrangements shows in the clearest manner 

 possible the influence of the spiral twist in the tube. All 

 can be explained by the movements made by the animal. It 

 is by thrusting itself by means of the uncini of the concave 

 side against the calcareous wall of the tube that it emerges 

 from it or retires. The greater activity of the organ of loco- 

 motion on this side has brought about their greater develop- 

 ment, and the viscera have been thrown to the opposite side. 

 This is especially obvious in the ovary j the ovules in course 

 of maturation are always on the convex side. 



We recognize that a natural (phylogenic) classification of 

 Sjnrorbis should have for its basis the direction of the twist 

 in the tube and that in each of the two series, right- and left- 

 handed, must be grouped the species provided with a fourth 

 setigerous thoracic ring on the one hand, and, on the other, 

 those in which this does not exist. From this results the 

 division of Spirorbis into four subgenera : — 



Right-handed 1 3 setigerous thoracic rings Dexioxpira. 



species j 4 „ „ „ Paradexiospira. 



Left-handed 13 „ „ „ Lo'ospira. 



species | 4 „ „ „ ParalcBospira. 



Lastly, we have been able to further define the phylogeny 

 of the group by the consideration of certain anatomical 

 characters, such as the form of the bristles of the first and 

 third setigerous thoracic rings, the modifications of the oper- 

 culum, especially in the cases in which it functions as an 

 incubating organ. 



On all these matters, and also for the new species we have 

 created, we would refer to our detailed memoir on these 

 animals. 



Note hy M. Edmond Perrieu on the above Communication *. 



The observations of MM. Caullery and Mesnil have 

 considerable interest from tiie point of view of the precise 

 * ' Comptea Rendus,' t. cxxiT. pp. £0-51. 



