BRITISH SPONGIAD.E. 67 



pletely developed specimen of P. mammillaris, or of one in 

 which the production of the fistulas may have been supposed 

 to have been arrested, and limited in number by adverse 

 circumstances, but a closer study of the two species renders 

 this idea inadmissible. The young of P. mammillaris always 

 has a more or less thickened base, and the incipient fistulas 

 are short, thick, and conical. A young specimen from Mr. 

 Barlee, dredged at Shetland, has the basal mass five lines 

 in diameter and two lines thick, and it has three such in- 

 cipient fistulas on its surface, the longest not two lines in 

 height, and no hispidation of the surface can be detected 

 with an inch lens. The young fistulas are developed pre- 

 cisely in the same manner on the circumference of a large 

 specimen of the same species, two and a half inches in 

 diameter, dredged at Orkney by Captain Thomas, and in 

 several other specimens of intermediate stages of growth 

 the same mode of development may be observed. We 

 may, therefore, presume, that this is the normal mode of 

 development of these organs in P. mammillaris. The same 

 mode also obtains in P. robusta. The aspect presented by 

 P. spinularia is very different. In a specimen sent to me 

 by the Rev. Walter Gregor from the Moray Frith, seated 

 in the hollow of a valve of a large water-worn Cardium, 

 the base of the specimen is oval, four lines by three and 

 not thicker than writing-paper. There has been two fistulas 

 produced, one has been broken off at the base, and the 

 other is quite perfect ; it is compressed in different directions 

 in its course from base to apex, but at its widest part it 

 does not exceed half a line in width, and is nine lines in 

 length, and the hispidation of its surface is comparatively 

 strongly produced ; while an adult fistula of P. mammillaris 

 measured one and a half lines wide and seven and a half 

 lines long. In other specimens from Shetland, in the 

 cabinet of the Rev. A. M. Norman, the length and breadth 

 of the fistulas are not so much out of proportion to those 

 of P. mammillaris, but they all have but two or very rarely 

 three fistulas, and in other respects exhibit a close alliance 

 with the specimens received from the Rev. Walter Gregor. 

 The general habit of this species is, therefore, strikingly 



