of Tetliyay/-om Shetland. 417 



jecting outwards, be evidently out of place ; and if these spi- 

 cules are to be considered liexradiate because a little projection 

 of the central canal may be observed where the sixth ray- 

 would be if developed, to carry out this principle in the 

 Spongiadffi will be found very inconvenient, if not wholly 

 impracticable. In distinguishing species, which is a purely 

 conventional arrangement, we should select, if possible, pro- 

 minent features that are easily recognizable, both for practical 

 purposes and to facilitate the study of natural history, 

 there being, comparatively, no limit to minute distinctions, 

 as there is no real line of demarcation in nature, if we do not 

 limit the power to which the microscope should be used in this 

 respect. The infinite mind of Nature does not require them ; 

 but the finite mind of man cannot get on without this aid, 

 and still less the " finite purse ; " when the more costly, ^. e. 

 the highest, powers of the microscope are required for their 

 detection. 



Tethya zetlandica^ n. sp. 

 PI. XXII. figs. 1-6 and 11-13 and 14-17. 



Conical, globular, or slightly compressed (PI. XXII. fig, 1). 

 Colour bright grey in spirit. Surface smooth, interrupted by 

 thick-set papilla irregularly disposed, large and separate 

 (fig. 2, a) or small and approximate (fig. 1, a). Pores and 

 vents for the most part closed by contraction. Internal struc- 

 ture consisting of bundles of spicules (fig. 13, hhh) radiating 

 from jfti excentric nucleus or point [a] to the circumference,where 

 they end respectively in the papillae of the surface ( /), im- 

 bedded throughout their course in the sarcode of the body {ccc)y 

 which is chai'ged, in the adult state, with minute ova (fig. 7), and 

 presents, in dilated cavities connected with the excretory canal- 

 system (fig. 14), a great number of pendent seed-like bodies 

 — that is, the young TetJtyce (fig. l^^ddd)'^ sarcode termi- 

 nating peripherally in a condensed tough lamina (fig. 13,eee), 

 which forms a kind of cortex to the whole, and, extending up- 

 wards on the projecting bundles of the spicules respectively , also 

 forms the papillary prolongations of the surface [fff)' Spi- 

 cules of three kinds, viz. : — 1, acerate slightly curved ; 2, tri- 

 forked, with the prongs of equal length ; 3, anchor-headed. 

 All these spicules vary in length with their position ; the 

 acerate, which are much the shortest, are chiefly confined to 

 the body and internal parts, while the two others chiefly oc- 

 cupy the surface and base, being shortest on the upper part of 

 the body and longest towards the base ; all three kinds may 

 be found projecting from the papillfe in variable plurality 

 when the anchors and forks have not been broken off, which 



