168 THE DEPTHS OF THE SEA. [chap. iv. 



encrusting sponge which we met with frequently 

 sticking to stones in the ' cold area.' The form of 

 the spicules was nearly though not quite the same, 

 and their arrangement was very similar. It appeared 

 as if the two forms placed in intermediate circum- 

 stances might have approached one another very 

 closely. 



In the warm area, as in the cold at these great 

 depths, there is a singular absence of Hydrozoa. A 

 few species of Sertularia and Plumularia, and one or 

 two allied forms occurred, and they are now in the 

 skilful hands of Dr. AUman for determination; but 

 their small number and insignificance is remarkable. 



Neither are the true corals represented by numer- 

 ous species, although in some places individuals 

 are enormously abundant. During the ' Porcupine ' 

 cruises of 1869 twelve species of Madreporaria were 

 procured which have been determined by Professor 

 Martin Duncan. None of these belong to 'reef- 

 building' genera, but to a group which are recognized 

 as deep-sea corals, a group which appears to have had 

 numerous representatives during all the later geolo- 

 gical periods. In a band somewhat restricted in 

 depth, extending downwards from the 100-fathom 

 line, we met in some places with very large numbers 

 of many varieties of Caryophyllia borealis, Fleming 

 (Fig. 4) ; and at depths of 300 to 600 fathoms the 

 handsome branching Lophohelia prolifera, Pallas 

 (Pig 30), forms stony copses covering the bottom 

 for many miles, the clefts of its branches affording 

 fully appreciated shelter to multitudes of Area 

 iwdulosa, Psolus squamatus, Ophiopholis aculeata, 

 and other indolent * commensals.' 



