MEKRILL: FOSSIL SPONGE SPICULES. 21 



takes the position that flints are formed by continuous growth of sponges 

 in situ for successive generations. But while he thinks this is true, 

 he accounts for the absence of the once existing flesh spicules by sup- 

 posing that they have all been dissolved. On page 459 of the same 

 paper he accounts for the peculiar forms of nodules by saying that 

 they indicate the irregular distribution of siliceous solutions about an 

 irregular bed of sponge spicules, at the time in which they replaced the 

 surrounding chalk and deposited silica in its interstices. The views of 

 these eminent men are doubtless the results of profound study and ex- 

 tensive observation, and are entitled to the most careful t consideration ; 

 therefore, before I comment on them, I desire again to remind the reader 

 that my observations are limited to a few nodules, and that I shall dis- 

 cuss the formation of these few rather than the formation of flint nodules 

 in general. 



It will be remembered that, in giving the method of preparation of the 

 flints for study, I stated that sections were cut through the centre and 

 also near the surface to And if possible any difference in the condition of 

 preservation of the spicules. The result of the study showed that, while 

 there was little difference in the preservation of the spicules in the body 

 of the nodule, yet it appeared that near the surface there was more 

 crushing and mechanical wear than further in the interior. Especially 

 was this true of the Monactinellid, Figure 7, which was more abundant 

 near the surface and was never found perfect. It should be stated, 

 however, that the mechanical crushing differed considerably in the differ- 

 ent specimens, and one showed complete obliteration of spicular structure. 

 In the sponge spicules studied by Carter, Sollas, and Hinde, all of which 

 had been subjected to mechanical movement, the smaller spicules had 

 all been destroyed. Carter says that he did not find minute stellates, 

 or spines, or tubercles on the large spicules in the Haldon deposit. 1 

 Sollas says that the once existing spicules are absent because they 

 have been dissolved. 2 Hinde says that flesh spicules are rarely met 

 with in the fossil state. 3 



Texas flint nodules, however, show a scarcity of zone spicules and a 

 great number of flesh and dermal spicules. Moreover, these minute 

 stellates, which are covered with exceedingly delicate spines, are per- 

 fectly preserved even to the most minute barb on a spine not exceeding 

 0.0037 mm. in length. It was noticed also that each slide had a num- 



1 Ann. Mag. Nat. Hist., 1871, Vol. VII. p. 118. 



2 Ibid., Ser. 5, 1880, Vol. VI. p. 442. 



3 Fossil Sponges of the British Museum, Introduction. 



