the Greensand compared lolth those of existing Species. 125 



to the Coralliosponglge, but also-wliicli of these, and of" those 

 with shafts in PL X., beh)nged to the different divisions in- 

 cluded under the head of Pachytragiffi. 



We muslf here, for reasons above stated, omit from these 

 altogether the Tethyada3, of which T. cranium is the type, 

 likewise Donatia {T. lyncurium) and its type, since, if 

 neither the trifid spicules of the former nor the stellates of the 

 latter can, from their extreme delicacy, be expected to be found 

 in the deposit, these species have no other spicular element by 

 Avhich they can be recognized with certainty. 



The thickness of the shaft at its base in the headed spicules 

 rather indicates a short than a long shaft, as is well known to 

 those who have studied the anchor- and vasiform trifid-headed 

 spicules respectively of the Geodida3 &c., for which compare 

 our fig. 63 with 59 in PI. X. ; but, as before stated, the ex- 

 pansion or elaboration of the head seems to take place at the 

 expense of the shaft ; and hence this so accords with what is 

 found in the Coralliospongise, that where there is no part of 

 the shaft left, and The thickness of the latter at its base is no in- 

 dication of length, the development of the head is our only guide. 



Thus, in the figures of PI. IX., I hnow 32 to be the end 

 view of the shafted spicule 66 in PI. X., because I myself 

 drew it from this spicule : and figs. 35 and 36 had also shafts; 

 but they are omitted because there was no room left in the 

 ])lates for lateral views of these spicules. Fig. 34 probably 

 had a long shaft ; and the head of 69, when viewed endwise, 

 presented a hexternate form somewhat like 33 ; but whether 

 figs. 30, 31, and 33 had short shafts or long ones there is 

 nothing to determine, as they are broken off close to tlie heads 

 in the fossils. 



Thus, while there can be little doubt of the heads which 

 have long shafts having originally come from some species of 

 the PachytragitE, I have no means of deciding whether figs. 30, 

 31, and 33 belonged to the latter or to the Coralliospongia^, since 

 nearly facsimiles of 31 and 33 are given by Dr. Bowerbank 

 in his illustrations of Dactylocalyx Masoni and D. Bowerhankii 

 respectively (Proceed. c«V.), and a facsimile of fig. 30 appears 

 in iSchmidt's illustrations of Stelletta {S. discoj^hora^ tab. iv. 

 fig. 5 a, Adriat. Spong. 1862). The latter also, in its hexra- 

 diate form, is no less characteristic of Wright's Wyville-'Thom- 

 sonia WaUichii (Quart. Journ. Microscop. Sc. Jan. 1870, pi. ii. 

 fig. 3), also Schmidt's Stelletta (Tisijjhonia) ayariciformis 

 (Atlant. Spong. Faun. Taf. vi. fig. 12), also DorviUia agarici- 

 formis, Kent (Monthly Microscop. Journ., Dec. 1870, pi. Ixvi. 

 fig. 7), and, lastly, my own figures of the spicules in Stelletta 

 asjjera and S. lactea (Ann. Nat. Hist. vol. vii. Jan. 1871). 



