318 SIDNEY F. HAT^MER. 



positiou, which is shown iu figs. 65, 66. From these it is 

 apparent that the modiau part of tlie fascia is exactly trans- 

 verse and vei'tical when the operculum is closed, and that on 

 each side it curves round in order to be attached to the 

 proximal end of the main sclerite. It thus results that while 

 not much more than the edge of the middle part of the fascia 

 is seen in a view from the basal or fi-om the opposite side, 

 the surface of the lateral parts is seen under the same con- 

 ditions. I have been unable to ascertain with certainty 

 whether the median part of the fascia is inserted into the 

 operculum or whether it has a free edge in this region. 



The oi'igin of the muscles can best be seen in a basal view 

 of a zooecium from which the basal wall has been removed in 

 the way recommended on p. 265. The hiteral recesses {I. r.) 

 are here seen to be almost entirely filled by four strong 

 muscles. Of these the distal occlusors [occl.') are far the 

 strongest, and they originate from the distal half of the floor 

 of the lateral recesses. The proximal or normal occlusor 

 [occl.) originates from the proximal half of the lateral recess, 

 in its outer part. The course of its tendons [tend.) is indi- 

 cated in fi^gs. 64 — 66. 



The distal occlusors, in helping to close the " orifice," 

 probably tend to pull the whole operculum towards the distal 

 end of the zooecium, since their fascia is inserted largely into 

 the projecting proximal ends of the main sclerite (figs. 65, 

 66). The proximal occlusors correct this tendency, as may 

 be inferred from the direction taken by their principal 

 tendons. When it is remembered that in this species each 

 of the strong teeth of the operculum fits, when closed, into 

 its own socket in the calcai'cous part of the zooecium, it is 

 obvious that the operculum must shut with great accuracy, 

 and a proper correlation between the contractions of the two 

 sets of occlusors no doubt has to be maintained. Tiie 

 accuracy of the closure is probably helped by the movement 

 of the basal sclerite over the median process, and, of course, 

 by the transverse ligaments uniting the operculum to the 



