68 A MONOGRAPH OF THE 



different from that of P. mammillaris. I deem it necessary 

 to be thus particular in the description of differential 

 characters as compared with P. mammillaris, as their struc- 

 tural characters appear to be so nearly allied, but a close 

 scrutiny and comparison of the spicula of the two species 

 exhibit sufficient differences to estabhsh their separate 

 identity on structural principles alone. Thus, the skeleton 

 spicula of P. mammillaris are larger, and in every specimen 

 examined the bases of the fusiformi-acuate spicula exhibit 

 more or less tendency to enormi-spinulation, while in those 

 of P. spinala no such character is visible. The defensive 

 spicula are as long as those of P. mammillaris but very 

 much more slender, and the whole texture of the dermal 

 surface more delicate and strongly hispid. 



6. Polymastia radiosa, Bowerbank. 



Sponge. Sessile, adherent through its whole length ; body 

 circular or oval, furnished with a single mammaeform 

 cloacal appendage; surface even, spiculous. Oscula 

 simple, dispersed. Pores inconspicuous. Dermal 

 membrane pellucid, furnished abundantly with small 

 short fusiformi-spinulate spicula matted together, and 

 also with large radiating groups of external defensive 

 sub-fusiformi spinulate, or sub-fusiformi acuate spi- 

 cula, large, and long. Skeleton. Longitudinal fasci- 

 culi few in number, large, dividing and anastomosing ; 

 spaces intervening, usually smaller than the diameter 

 of the fasciculi; spicula acuate, occasionally sub- 

 spinulate, large, and long. 



Colour. — White. 



Habitat.— ^]\ei\&x\A and Peterhead, N. B., Mr. C. W. 

 Peach. 



Esoamined.— In the dried state. 



I have received two specimens of this species from my 

 friend Mr. C. W. Peach. One found at Peterhead, and 

 the other dredged at Shetland, in 1864. The form and 



