REPORT ON THE MOLLUSCA. 19 



jeffreysi the gills are free and the pallial cavity almost undivided, in Lt/onsidla 

 papyracca the gills are united to the mantle and to each other, and are disposed as in 

 Lyonsia (PL II. fig. 9), except that the anal chamber is larger, ventrally as well as 

 anteriorly. 



But my astonishment was still greater when, on studying two specimens of the 

 typical species Lyonsiella ahyssicola, Sars (which were sent to the Challenger Office by 

 Professor G. 0. Sars), I ascertained that the smaller of the two specimens (about 

 2 mm. in length) exhiliited the arrangement observed in Lyonsiella jeffreysi, and 

 that the larger (about 4 mm. in length) was similar in this respect to Lyonsiella 

 papyracea (compare PI. II. figs. 8, 10, and PI. III. figs. 1, 2). 



From the very succinct description given by Sars, and from his figure {loc. cit., PI. 

 III. fig. 42), one cannot make out whether the gills are free, as in Lyonsiella jeffreysi, 

 or united to form a partition, as in Lyonsiella j3aj)_(/racea. As, except in the arrange- 

 ment of the pallial " chambers," the structure of the two specimens of Lyonsiella 

 ahyssicola which I have studied is exactly similar, I shall here complete Sars' account 

 by giving two figures (PI. II. fig. 10, and PI. III. fig. 2), and a short description of the 

 larger specimen. 



The mantle is formed as in the preceding species. The pallial commissure separat- 

 ing the pedal and branchial apertures is very delicate. As in Lysonia, there is a small, 

 very short retractor muscle of the siphons at eaeh side. 



The fold surrounding the mouth is so disposed that one can observe a more 

 marked separation between "lips" and "palps." In fact, it forms four lateral lobes 

 corresponding to palps (PL III. fig. 2, b and c) ; an anterior pair, more developed, and 

 a posterior pair (c), extending on each side of the foot (d). 



The gill ((? and e') arises between the two lobes on each side. It extends from the 

 anterior adductor to the division between the two posterior apertures (branchial, q, and 

 anal, p). The two gills are situated almost in the same longitudinal plane (see PL II. 

 fig. 10, e). Their outer plate (PL III. fig. 2, e') is less developed than the inner one. 



The two gills are joined together behind the foot. They are united to the mantle 

 on each side and posteriorly, at the separation of the two siphons, by the membrane (h). 

 Thus an " anal" chamber (PL II. fig. 10, o) is formed, dorsal to the partition made by 

 the gills and the membrane (h). 



As I have not been able to examine a larger number of specimens of the genus 

 Lyonsiella, I can only suppose, from the four individuals studied, that the arrangement 

 observed in the large specimen (described and figured) of Lyonsiella ahyssicola does not 

 exist in the young stage ; and that certain species of the genus (Lyonsiella jeffreysi) 

 always preserve the arrangement of the young stage, while others have two entirely 

 separated pallial chambers in the adult stage. 



