356 THE VOYAGE OF H.M.S. CHALLENGElt. 



to be contrasted with the corresponding nodes of most other Dictyonina in possessing, as 

 a rule, no freely projecting radial ray, but simple slightly arched extremities (PL CIII. 



fig- 3). 



The dermal skeleton contains h5q5odermal hexaets sometimes mixed with pentacts, 

 and provided with smooth rays which are occasionally roughened at the extremities. Of 

 these the distal ray, which is absent in the pentacts, is shortest, often simply rounded or 

 even terminating in a small swelling, while the four tangentials which run out to a 

 point, and the much longer proximal ray always taper to a point. On the outer 

 extremities the hexact hypodermalia support a floricome-like discohexaster in which 

 each of the short principal rays bears seven (more rarely fewer) S-shaped terminals, 

 which run out externally into a thickened hemispherical transverse disc with a notched 

 margin (PI. CIII. figs. 7, 8, 9). The spicules which constitute the gastral skeleton have 

 an exactly similar form and disposition (PI. CIII. fig. 3). 



Scattered throughout the dictyonal framework of beams I have also to note small 

 simple hexaets with somewhat rough uniformly tapering rays (PL CIII. fig. 4), and 

 oxyhexasters with short principals and long S-shaped pointed terminal rays (PL CIII. 

 figs. 5, 6). Since there are two or three terminals on each principal ray, and since each 

 of the six principals may either remain simple and undivided, or may be divided into 

 terminals, all transitions occur between a hexact with five undivided somewhat rough 

 rays and only the sixth ray forked, and an oxyhexaster in which each of the principal 

 rays is divided into three terminals, giving a total of eighteen (PL CIII. fig. 5). 



Genus 5. Aulocystis, n. gen. (PL CIV.). 



History. — In his sponge-system Gray^ added to the description of Myliusia callo- 

 cyathus, the following brief notice : — " There are two smaller specimens in the British 

 Museum which probably belong to the same species. The smaller one was collected by 

 the Rev. L. Guilding at St. Vincent in 1840, and the other was received from the West 

 Indies by Mr. Scrivener in 1842." 



These sjDecimens, which were not further studied b}' Gray, were subsequently 

 examined by Bowerbank, who, in 1869,^ gave the following account of them. While he 

 maintained that the specimen procured by Mr. Scrivener in 1842 agreed in structure with 

 Myliusia {Iphiteon, Bowerbank) callocyathus, Gray, " he found that the specimen collected 

 by the Rev. L. Guilding at St. Vincent difi'ered widely in the structure of its skeleton 

 from either Ipliiteon, Bowerbank, or Dactylocalyx. He proposed, therefore, to apply 

 Dr. Gray's generic title Myliusia to this species, instead of to Myliusia {Iphiteoit, Bower- 

 bank) callocyathus, Gray. He accordingly named the form Myliusia grayi, and briefly 

 diagnosed it as follows : — " Sponge sessile, massive. Dermal surface unkno'RTi. Surface 



' Proc. Zool. .S'oc. Loiul, 1867, p. OdC. ^ p^oc. Zool. Soc. Luwl, 1S69, p. 335. 



