142 PROCEKDINGS OF THE MVLACOLOGICAL SOCIETY. 



the microscope, a most minute pigment spot was discovered on each 

 tentacle in the position indicated above. 



Projecting from the mantle cavity on the right side of the body, 

 and even curving round the margin of the shell when the animal 

 is fully extended and actively moving, is a large pectinate gill. This 

 somewhat exceptional character was noted by Duprey, but Fischer 

 neither figures nor describes it, his specimen being evidently in 

 a moribund condition. The relations of the gill at first sight recall 

 that of Valvata, but in this latter genus the gill does not extend 

 jDosteriorly into the mantle cavity for any distance, whereas in 

 Adeorhis the gill is well developed posteriorly on the dorsal wall 

 of the mantle. 



Two small tentacles are to be observed on the right side behind 

 the gill (PL YIII, Fig. 1). These tentacular appendages of the 

 mantle are in the position of a posterior siphon ; they are not, 

 however, grooved prolongations of the mantle margin, but solid, 

 tentacular outgrowths. Both Fischer and Duprey appear to have 

 overlooked these appendages, which strongly recall the single, similarly 

 placed, tentacular thread of Valvata (Bernard [5] ). Some species of 

 Rissoia also possess such a structure which may be present on both 

 sides of the body. Right and left tentacles in this same position 

 also occur in Oliva and in the embryo of Vivipara. I^one of these 

 forms, however, show two tentacles situated close side by side. 



Internal Anatomy : The Alimentary Canal. — The mouth is armed 

 laterally with a pair of little plate-like horny jaws, composed as in 

 many Tsenioglossa of minute chitinous tesserae. The buccal mass is 

 fairly large and muscular, and possesses a well -developed radula-sac. 



The radula has been described and figured by Fischer [4], but as 

 neither his figure nor his description is quite accurate, I have thought 

 it advisable to refigure the same. The most striking feature (PI. VIII, 

 Fig. 3) is the lobate posterior border of the base of the central tooth. 

 An examination of a very large series of radulge, such as we find 

 figured in Troschel's "Das Gebiss der Schnecken" [7], would suggest 

 that this is a very uncommon character, only to be met with elsewhere 

 among the Bissoiidse ; and it is with Rissoia that Fischer compares the 

 radula of Adeorhis. When comparing the central tooth of Adeorbis 

 with that of Rissoia, if we were to restrict ourselves to Troschel's 

 figure of the latter, we should be compelled to admit that Fischer's 

 comparison was a very just one ; but happening to procure a number 

 of live specimens of Rissoia parva, for the purpose of instituting 

 a comparison of the soft parts with those of Adeorhis, I made 

 a preparation of the radula, and found that Troschel's figure of the 

 central tooth was not quite accurate. The posterior border of the base 

 of the central tooth in this form (PL VIII, Fig. 4) is not cleft, but 

 entire, save for two slight indentations. It is, however, at once 

 obvious where Troschel made his mistake : there are on the base of 

 this tooth two pairs of small, upstanding, backwardly directed 

 denticles, a larger inner and a smaller outer pair, very similar to 

 those seen in Bythinia and Paludestrina ; these Troschel seems to 

 have mistaken for the posterior border of the tooth, which latter 



