Mr. W. H. Benson on the Characters of Streptaulus. 201 



a curved figure, or half-cylinder. The mode of action of the gill 

 during the respiratory act is precisely the same as in the case 

 of the Octopus. The water enters and penetrates in the same 

 direction ; it escapes expiratorily in the same manner. In all 

 other respects the branchiae in these two examples are con- 

 structed in precise accordance with the same principle. There 

 is no deviation ; in minute structure they are the same. 



EXPLANATION OF PLATE XV. 



Fig. \. The left gill of Octopus vulgaris, in situ: a, efferent vessel or 

 branchial vein ; b, afferent vessel or branchial artery ; c, branchial 

 heart; d, d, secondary lobuli of the branehia;' c, c, tertiary 

 leaflets ; /, the hindmost lobule, showing that the axis of the 

 gill is closed both behind and anteriorly at ^ ; h,h, the sub- 

 branchial longitudinal muscle. 



Fig. 2. Transverse section of the gill (effected through the interlobular 

 space) : a, branchial vein (in section) ; b, branchial artery ; c, lon- 

 gitudinal sub-branchial muscle (in section) ; d, attachment to 

 mantle ; e, e, g, g, secondary lobular artery (or afferent vessel) ; 

 /, the fraenum or membrane which forms a framework and sup- 

 ports the branchial foliage, i, i, i, i ; h. represents the water-spaces 

 between the lobuli. 



Fig. 3-3'^. Gill of the comniou Calamary, viewed from the dorsal side ; 

 3^, the same, viewed from the ventral side : a, a, secondary lobuli ; 

 b, b, afferent primary and secondary vessels ; c, the tertiary lobuli. 



Fig. 4. Transverse section of the gill of the Calamary : a, efferent vessel, 

 in section; b, afferent ditto; c,c, secondary efferent trunks; 

 d, d, secondary afferent ditto ; e, e, e, tertiary foliage ; /,/, water- 

 spaces ; g, g, vessel ; h, fraenum by which the gill is attached to 

 the mantle. 



Fig. 5. Plan of ultimate vessels. 



Fig. 6. Plan of the same at free border. 



XVI. — Characters of Streptaulus, a new genus, and of several 

 species of the Cyclostomacea from Sikkim, the Khasia Hills, 

 Ava, and Pegu. By W. H. Benson, Esq. 



Streptaulus, Bens. Genus novum. 



Testa umbilicata, pupiniformis, nitens ; peristoma circulare, non con- 

 tinuum, superne tubule suturali interno et externo, continuo, ad 

 extremitates ambas aperto, siphonem mentientc, perforatum. 

 Operc ? 



Streptaulus Blanfordi, Bens., n. s. 



Testa umbilicata, oblonga, polita, rcgulariter oblique striata, striis 

 prope suturam submargiuatam fortioribus, lineis uomiuUis spirali- 

 bus decussatis, fusco-cornea, translucente ; apice obtusiusculo ; 



