— 47 — 



curved, of which three are disposed nearly in a plane or 

 forming a very depressed pyramid. One arm ot these three 

 is frequentl)- longer than the others, slightly curved, and at 

 the extremity bifurcates into two small irregular rays (fig. 32, 

 33). The arm at the apex of the pyramid is short and obtusely 

 blunted and in some instances is continued beyond the apex 

 of the pyramid, so that a five-armed spicule is the result, 

 (figs. 28, 34), As in other respects these five-armed spicules 

 correspond closely with the four-armed, I am disposed to consider 

 them as mere variations and not as indicating a different species. 

 The length of the arms of these spicules varies between 0,495 "'''"• 

 and 1,125 Tim. with an average thickness of 0,14 mm. The 

 examples of these quadrifid spicules are but rarely met with. 



These spicules correspond so closely with those of 

 Pachastrella primoeva, Zittel, from the Upper Chalk of Ahlten 

 in Hanover, that, so far as can be judged from the zone 

 spicules , they may have belonged to the same species. In 

 the Ahlten sponge, the larger spicules are also accompanied 

 by great numbers of the smaller simple quadrifid spicules 

 .similar to those of my fig. 25, and these smaller forms both 

 in the fossil and recent sponges seem common to different 

 species of Pachastrella. I have also seen a specimen of Pachastrella 

 from the Flamboro' Chalk, given by Mr. Carter to Professor 

 Zittel, in which the furcation of the skeleton spicule is carried 

 to a greater extent than in the Ahlten and Horstead examples, 

 and the spicule is also less robust. In the furcation of the 

 extremities of the spicule, the Chalk examples correspond 

 closel)- with the skeleton spicules of Pacliastrclla abyssi, 

 O. Schmidt, and also with P. intertexta. Carter. (An. Mag. 

 Nat. H. S. 4. Vol. 18. p. 409 PL XV. fig. 41) dredged from 

 the Atlantic near Cape St. Vincent at a depth of 374 

 fathoms. Examples of the five-armed spicules also occur in 

 P. intcj'tcxta and in P. parasitica, Carter (An. Mag. Nat. Hist. 

 S. 4. Vol. 18. p. 410 PL XVI, fig. 50.) 



