Diagnoses of the families, genera and sI'Ecies. 109 



The family I divide into two subfamilies, viz.: 



1. Euplectellinœ. Euplectellidse rooted in the substratum by 

 a tuft of basal spicules. 



2. Corbitelllnce. Euplectellidse firmly attached to the sub- 

 stratum by a compact base. 



To the latter belongs 



PLACOSOMA Nov. Gen. 

 With one species. 



Placosonia jfff^'ndicffjmiff h- g-, n. sp. — Corbitellinœ with 

 laterally compressed, massively developed, soft body and moderately 

 long, firm stalk. On top, a comparatively small primary osculum 

 leading into the shallow gastral cavity. One side (the front) of 

 the body is covered with a regularly quadrate-meshed, dermal 

 latticework which is supported on another latticework composed of 

 hypodermal fibrous bundles ; the other side (the back) presents 

 a more dense-looking surface in which open a large number of 

 secondary oscula leading into the excurrent canal-system. Paren- 

 chymalia, principally diactins and occasionally hexactins. In 

 certain positions on the front the dermalia may have tho proximal 

 ray so reduced in length as to be not longer than the short 

 distal ray. Gastralia, hexactins, Hexasters in three varieties : 

 the hexactinose discohexaster (30-60/^ dia.), smallest and most 

 numerous; the spherical discohexaster (lGO-240/^ dia.), large 

 and extremely beautiful, confined to the back side of the sponge; 

 and the hexactinose codonhexaster (110-17(3« dia.) found under 

 the gastral layer. Floricome and graphiocome, not present. 



