138 THE VOYAGE OF H.M.S. CHALLENGER. 



a double row of minute, spiny, fusiform spicules, which are to be found in the outer 

 walls of each of the tentacles, which, when the tentacles are withdraw i, form a protection 

 to the soft parts of the polyp. 



Measurements of the Spicules. — The large spindles l-0"3 ; r6-0"5 mm. The 

 smaller spindles 0-6-0 ;2; 0-5-0-1; 0-42-0-1; 0-4-0-14; 0-3-0-08 mm. The clubs vary 

 from 0'3 to 0'2 mm. in length. Those with broad folia measure in the broadest diameter 

 0'15 mm., those with narrow folia 0"08 mm. The four-rayed spicules measure 0"1- 

 O'l mm. The needle-shaped spicules at the base of the tentacles measure 0"2-0'02 ; 

 0-06-002 mm. 



Habitat. — Bermuda ; shallow water. 



Genus Plexauroides, n. gen. 



Colony branching, mostly in the one plane. Stem and branches cylindrical. Axis 

 horny, dense, with the central portion calcareous. Nutrient canals symmetrically 

 arranged. Coenenchyma thin, consisting of two very distinct layers of spicules, the 

 one nearest the axis is composed of irregular stellate forms, the outer layer of large 

 Blattkeule, the broad foliar expansions of which project beyond the surface of the 

 coenenchyma, forming a rough imbricated surface. 



The polyps are numerous, fully retractile, with scarce!)^ visible verrucae, the edges of 

 these latter being fringed with rows of the broad projecting folia of the Blattkeule. 



This genus is established for the forms of Plexaura with large Blattkeule, which 

 project by their thin-edged folia beyond the surface of the coenenchyma, and present the 

 appearance of a series of imbricated scales. 



On a revision of all the genera of the Plexauridse it is probable that several species 

 of Plexaurella may have to be relegated to this genus. 



Plexauroides prselonga (Ridley) (PI. XXVIII. figs. 1, la; PI. XXXIII. fig. 2). 



Plexaura prselonga, Eidley, Rep. Zool. CoU. B.'M.S. "Alert," p. 339, 1884, pi. xxxvi. fig. F, 

 pi. xxrviL figs. g. g'. 



Two specimens, which may be referred to the above species, are in the collection 

 from Torres Strait. 



The colony is erect, branching in one plane, forming an irregular dichotomy. In 

 one of the specimens (that figured) the colony is attached by a flat, irregular disc, 

 measuring 15 by 10 mm. The main axis, at about a height of 20 mm., gives off a 

 branch 30 cm. long; then at an interval of 10 mm. a second branch nearly as long as 

 the first ; at a distance of another 10 mm. a third branch arises, but from the opposite 

 side of the axis, this one reaches a length of 20 cm., while the main stem continues 

 unbrauched for a further length of 26 cm. 



