Miscellaneous. 583 



MISCELLAXEOUS. 



On the Organization and Affinities of Pleurotomaria. 

 By MM, E.-L. Botjviek and H. Fischer. 



The Pleurotomarice are the first mollusks (one miglit almost say 

 the first animals) of -which remains are found in the fossil state. 

 As old as the oldest Trilobites, but curiously more tenacious of 

 existence, they were not extinguished like the Trilobites during the 

 Carboniferous epoch. They have left a great variety of forms in 

 the Jurassic deposits, they were perpetuated, though somewhat 

 feebly, through the Tertiary period, and are still represented in our 

 day by certain species, for the most part of extreme rarity. Of these 

 last absolutely nothing was known but the shell until the ' Blake,' 

 under the direction of Alexander Agassiz, captured four living 

 Pleurotomaria; off the Antilles in 1879. From these specimens 

 M. Dall was able to determine that the anus is still situate on the 

 median dorsal line in the animals we are considering, that they have 

 two branchiae symmetrically situated, two urinary orifices also 

 symmetrical, and that thus they present, as one might expect, by 

 far the most primitive characters of all Gasteropods. Since that 

 time the Japanese have found another living Pleurotomaria ; but it 

 was not made the subject of any research, so that the internal 

 organization of these creatures might have remained for a long time 

 unknown had not M. Agassiz, thanks to the courteous intervention 

 of M. Milne-Edwards, handed over to us one of the specimens of 

 Pleurotomaria Quoyana collected by the ' Blake.' We cannot suffi- 

 ciently express our gratitude to the distinguished American zoologist 

 for his liberality. 



From its general organization P. Quoyana approaches most 

 nearly to the Diotocardiae of the most normal type (Haliotidae, 

 Trochidae). Its sense-organs occupy the same position ; its buccal 

 mass appears to be built on the same type ; it has the same myology, 

 the same cerebroid and buccal ganglia, the same labial commissure, 

 the same crossed (croisee) visceral commissure. 



The characters which distinguish it from the other Diotocardiae 

 are : — (1) the feeble development of the epipodium ; ^2) the alto- 

 gether peculiar origin of the branches of the visceral commissure ; 

 (3) the structure of the scalariform nervous cords which traverse 

 the foot. 



Of the " epipodial collarette " we will say nothing except that, 

 greatly reduced as it is in general in Pleurotomaria, it is imper- 

 ceptible in our specimen, and there is every reason for believing 

 that in the genus under consideration we see it at the very be- 

 ginning of its development. 



As for the " branches of the visceral commissure," they are not 

 in any wise detached, as in the other Diotocardiae, from the most 

 anterior portion of the nerve-cords situate in the foot ; they arise 

 from cerebro-pallial links towards the middle of their length, and 



