1908. J ASPIDOBRAXCH GASTROPOD MOLLUSCS. 825 



cavite branchiale et s'ouvre a sa base clans le sinus sanguin que 

 nous venons cle signaler." Further on (p. 312) he discusses the 

 homology of this organ and suggests that it may represent the 

 right ctenidium or the right osphradium, but gives no decided 

 opinion on this point. A description and discussion of the 

 significance of this organ will be found on p. 864. 



Though he made, as he tells xis, a careful search for it, Lenssen 

 failed to discover the supra-intestinal nerve ; but being cautious 

 he does not venture to affirm that it does not exist. Nor am I 

 ready to deny its existence, but after searching most carefully 

 through sevei-al series of sections I am unable to discover a trace 

 of it ; and it is very possible that this nerve, extremely small in 

 Nerita and the tropical species of Neritina, has actually dis- 

 appeared in K.fluviatilis. In the descriptive part of this paper 

 I shall have something to add to Lenssen 's account of the left 

 branchial ganglion and the osphradium. It is not necessary for 

 me to refer at length to Lenssen's account of the circulatory, 

 respiratory, and excretory systems. Though somewhat short, his 

 descriptions of these systems are accurate so far as they go, and 

 he is the first author to give a true and intelligible account of the 

 kidney and reno-pericardial duct. 



I make no sej)arate reference to the literature bearing on the 

 anatomy of Septaria [Navicella). This genus has not been studied 

 in detail by any author, but Bouvier, Boutan, and others have 

 described the nervous system in the works already quoted. As 

 my interest in the Neritacea dates from some dissections of 

 Sepiaria which I made for the purposes of my class, and as the 

 secondary symmetry acquired by this genus makes it a very 

 favourable object for describing and figuring the somewhat 

 complex relations of the coelom and genital ducts in the ISTeritacea, 

 I will begin the account of my own work with a description of its 

 anatomy. 



Genus Septaria Ferussac. 



The species of this genus available for my researches were 

 S. horhonica Bory, ^S'. dejJressa Lesson, both forming part of the 

 collections of the Oxford Museum, and S. hougainvillei Recluz, 

 from the British Museum. The number of specimens at my 

 disposal was small, and I unfortunately dissected the only two 

 specimens of Septaria horhonica that I possessed before I had 

 made myself thoroughly familiar with the problems of Keritacean 

 anatomy. A specimen of S. depressa was cut in horizontal and 

 one of S. hougainvillei in transverse sections. Both these 

 specimens proved to be females. There are some minor points of 

 difterence in the anatomy of the two species which will be referred 

 to in due course. 



A dorsal view of S. horhonica is given in fig. 1 *. The roof 



* The figure-numbers 1-69 in this Memoir refer to the fig'ures on Plates XLVI.- 

 LXVI., which are described on pp. 885-837. There is only one text-figure (text- 

 fig. 172, p. 855). 



