Mr. H. J, Carter on known Fossil Sponges. 287 



the unbroken end to the centre of the bow doubled has to be 

 taken for the total length, wliich is l-7th inch, as above 

 mentioned, while that of the other largest specimen, which is 

 perfect, is l-16th in. There are several specimens in the 

 piece of chert mentioned which are very near together, and 

 being close to the surface of the fractured portion of the chert 

 are satisfactorily seen, while the bow in some of the smaller 

 ones appears to be higher and wider, *'. e. more like that of 

 Microciona armata. 



Doubtless hereafter there will be more fossilized spicules 

 found which can be identified with those of recent Mon- 

 axonida ; but at present they are all confined to what I have 

 delineated, with the exception of the tricurvates just men- 

 tioned and what have been added by Prof. Zittel and Dr. 

 Hinde in their works respectively, viz. those by the former 

 in the Abhandl. der k. bayer. Akad. der W. ii. CI. xii. Bd. iii. 

 Abth. Taf. iii.-vii., and those by the latter in his Mon. 

 Brit. Foss. Sponges (Palajont. Soc. Publ. vol. for 1886, pt. i. 

 p. 66). 



We now come to my seventh order, viz. the Hexacti- 

 NELLIDA, which, from Salter's Protospongia fenestrata in the 

 Cambrian, have been continued down to the present day, 

 manifesting themselves plentifully in a Hyalonematoid form 

 from the Carboniferous Limestone of Ayrshire, viz. Hyalo- 

 nema Smithii (' Annals,' 1878, vol. i. p. 129, pi. ix. figs. 1- 

 14), subsequently called '■'■ Hyalostelia'''' by Zittel (Hinde, 

 Cat. Foss. Spong. Brit. Mus. p. 150, pi, xxxii. figs. 1, 1^). 

 But it is not until the Cretaceous period is reached that the 

 vitreous Hexactinellida or so-called '' Glassy Sj^onges " 

 appear to have come into prominence, and here their maximum 

 of development, like that of the Lithistida, seems to have taken 

 place, as may be seen by a reference to Zittel's illustrated 

 description of this order (Abhandl. der k. bayer. Akad. der 

 W. ii. CI. xiii. Bd. i. of 1877, translated by Mr. Dallas into 

 the ' Annals ' of that year, vol. xx. pp. 237, 405, and 501 ; 

 also Dr. Hinde's illustrated description of the Fossil Sponges 

 in the British Museum, p. 91 &c.). 



Among the detritus of the Upper Greensand in this locality 

 to which I have alluded, the remains of the Hexactinellida 

 that I have found are very scarce, in comparison with those 

 of the Lithistida, so that we may fairly infer that the former 

 were not so plentiful as the latter, as shown by ZittePs splen- 

 did researches (?. c.) . While at the present day they appear 

 to bear a similar proportion, so far as my observations extend, 

 which would have been more complete had I been able to refer 



