My. 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, which is 1-7th inch, as above 
mentioned, while that of the other largest specimen, which is 
perfect, is 1-16th im. 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. 1. Cl. xii. Bd. iii. 
Abth. Taf. iii—vil., and those by the latter in his Mon. 
Brit. Foss. Sponges (Paleont. Soc. Publ. vol. for 1886, pt. i. 
». 66). 
We now come to my seventh order, viz. the Hrexacti- 
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. ‘yalo- 
nema Smithit ( Annals,’ 1878, vol. 1. p. 129, pl. ix. figs. 1- 
14), subsequently called “ Hyalostelia” by Zittel (Hinde, 
Cat. Foss. Spong. Brit. Mus. p. 150, pl. xxxii. figs. 1, 1g). 
But it is not until the Cretaceous period is reached that the 
vitreous Hexactinellida or so-called ‘Glassy Sponges ”’ 
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. Cl. 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 Zittel’s splen- 
did researches (/. ¢.). 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 
