438 " THETIS " SCIENTIFIC RESULTS. 



in what must be approximately their natural position, as also do 

 patches of tissue, so that I am enabled to give the sequence of 

 tissues (it is, however, quite out of the question to attempt to 

 describe them), as well as describe the position of the organs. 



The wall of the proboscis is made up of five layers of tissue, 

 they are cuticle ('), deric epithelium (•^°), circular muscle strands 

 (^), a layer composed of connective tissue, diagonal and longitu- 

 dinal muscle strands (-), and the innermost layer, longitudinal 

 muscle strands gathered into bundles (■^). 



The rachi/ar-sac occupies the greater portion of the cavity of the 

 proboscis anteriorly; its wall consists of, from without inward, cir- 

 cular muscle strands, longitudinal muscle strands, columna epithe- 

 lium of which the cells are of medium length, and a layer of cuticle. 

 No sign of coelomic epithelium was found, nor was it expected. 

 The odontophore cartilage is saddle-shaped, but the " flaps " 

 are thicker than the "seat." Owing to the general destruction I 

 was unable to see the arrangementof the tissues of the odontophore. 

 There seems to have been a band of transverse muscle strands 

 across the convex side of the seat ; down the sides there was 

 apparently a layer of longitudinal strands, and outside this 

 a layer of short epithelial cells. 



The oesophagus is placed dbrsally to the radular sac, and lies 

 just under and attached to the dorsal wall of the proboscis. Its 

 wall, which is much folded inwardly, consists of a layer of cii'cular 

 muscle strands (^), one of longitudinal strands (**), enteric epi- 

 thelium (''), and a thick layer of cuticle {^^). 



The salivary ducts {^ and ■'), lined with a flat epithelium, are 

 situated in two isthmuses which extend into the lumen of the 

 oesophagus ; they are imbedded in connective tissue and longi- 

 tudinal muscle strands. 



IV.— ANATOMY OF THE GENUS. 

 1. External features. 



The shell. As in the specific descriptions, so in the generic, I 

 must refer my readers to descriptions already published ; these 

 will best be found by reference to Hedley's papers previously 

 quoted. 



The protoconchs differ in the two species, but these differences 

 are of degree rather than of kind, and both are referable to the 

 one type, which may be termed the Perostylus protoconch. 



Opjerculiun fusoid, nucleus anterior, component rings slightly 

 imbricating. 



Theybo^, as far as may be judged from contracted spirit speci- 

 mens, is large, and bears along the anterior margin a well- 

 developed mucous (?) groove. 



