SPIRACTINELLA. 1C5 



tinellid spicules have tapering, pointed rays ; in the modified compound forms the 

 rays are once, or oftener furcate, so that in certain examples they resemble stellate 

 spicules. The surface of the rays of both the larger and smaller spicules consists 

 of a spiral ridge extending from their bases to their tips. In the bases of the 

 larger spicular rays the ridge is broken up into a series of ellipsoidal nodes. 



I propose this genus to include the forms placed by Mr. H. J. Carter, F.R.S., in 

 Holasterella Wrightii. The spicules so markedly differ from those of the type form 

 of Holasterella, as far as these latter can be ascertained, that they may be regarded 

 as belonging to a distinct genus. In the simplest spicules there are six straight, 

 pointed rays at right angles to each other, and their surfaces are ornamented with 

 a continuous spiral ridge or coil; in some of the compound forms the rays 

 bifurcate equally near their bases, and the spicules then consist of twelve sub-equal 

 rays. Most of the larger spicules are of this type. In the more complex stellate 

 spicules the primary six rays appear to be always present, but they divide and sub- 

 divide near their bases somewhat irregularly, so that from each primary ray three, 

 four, five, and even six rays are given off in such a manner that the spicule has a 

 star-like form, and consists of a variable number of rays, ranging from twelve to 

 thirty- six. 



The spicules of this genus are characterised not only by their spiral coil, but by 

 the furcation of the rays of the larger skeletal-spicules as well as of the smaller 

 stellates. Only a single species is at present known, and this was discovered by 

 Mr. J. Wright, F.G.S., in decayed chert of Carboniferous age. 



43. Spieactinella Weightii, Carter sp. Plate VIII, figs. 1, 1 a — 1 h. 



1S80. Holastebella Weightii, Carter. Ann. and Mag. Nat. Hist., ser. 5, 



vol. vi, pi. xivB, figs. 1 — 7. 



1883. — — Hinde. Cat. Foss. Sponges, p. 153, pi. xxxii, 



figs. 4—4/. 



The character of the spicules has already been stated in the definition of the 

 genus. In the larger spicules the rays do not appear to divide more than once, 

 they diverge from each other near the base of the simple ray at an angle between 

 70° and 80°. No complete large spicule has yet been met with ; judging from 

 imperfect specimens all the rays appear to have been furcate. The secondary rays 

 of these larger spicules attain in some cases a length of 2 mm. by '5 mm. in thickness. 

 The spicules with six simple rays appear to be all of intermediate size, in these the 

 vertical axis is usually longer and more tapering than the transverse axes ; in an 

 average form the longer axis measures 1*2 mm. by '15 mm. in thickness. The 



Y 



