70 Prof. P. M, Duncan on the Salenidffi. 



single tubular prolongation (which, as the Carpenteria in- 

 creases in size, becomes branched and thus multiplied), it 

 also becomes even more subdivided than the branching in 

 Pohjtrema. 



What, therefore, takes place in the tubular prolongation of 

 the aperture in G. monticularis may also be the case in G. 

 balamformis and in G. [Polytrema) utricularis ; but being so 

 delicate that the slightest force almost breaks it off, there is 

 very little chance of a specimen of either coming into our pos- 

 session with the tubular prolongation even in the imperfect 

 state of that above mentioned ; hence the rarity. While in 

 the living state, where broken off, the margin becomes 

 smoothed down, rounded, and thus inflated by the animal, 

 which form presents a marked contrast with the jagged ap- 

 pearance occasioned by fracture after death. Thus a natu- 

 ral and a fractured margin may be readily distinguished. 

 When the tubular prolongation of either Polytrema or Car- 

 penteria is preserved, it will probably be found to arise from 

 the specimens being situated in depressions which have pro- 

 tected them from coming into contact with such force as must 

 have broken it off; for the tubular prolongation of the em- 

 bryonic Polytrema cannot be touched with the point of the 

 finest hair-pencil without risk of breaking ; and therefore, 

 when found in a dusty state (which is generally the case), no 

 attempt to clean it in this or any way should be made, or the 

 probability is that the delicate extremity, with the few sponge- 

 spicules that generally project from it, will be destroyed. 



VIII. — On the Salenidse, Wright. — Part I. Observations on 

 the Morphology of a Recent Salenia. By P. Martin 

 Duncan, F.R.S., Pres. Geol. Soc. 



[Plate II. B.] 



Having been lately engaged in studying the comparative 

 morphology of the group of Salenidse, some interesting and 

 rather important points in the structure of a recent specimen 

 probably referable to Salenia varispina^ A. Agassiz, have come 

 to light. 



The Salenidse, according to Wright, form a natural family 

 of the Echinoidea Endocyclica, their characteristics being 

 " the peculiar structure and great development of the apical 

 disc, which, besides the five genital and five ocular plates, has 

 an additional or suranal plate developed in the centre of the 



