Srom the Middle Lias of Northamptonshire. 355 
The figures of the spicules on the accompanying Plate 
(Pl. XVIL. figs. 7, 8,9), which have all been drawn to a 
uniform scale of 200 diameters, will convey a better idea of 
their relation in size and form to those of recent Calcisponges 
than a verbal description. Judging by the standard proposed 
by Heckel (Kalkschw. Bd.i.p. 209) for recent spicules, these 
fossil forms are included in the four lowest grades of minute, 
small, medium-small, and medium-large forms; that is to 
say, the rays of the smallest fossil three-rayed spicule are 
only -03 millim. in length, and thus within the sixth or lowest 
seale, whilst the rays of the largest: observed are -26 millim. 
in length, and thus of the fourth or medium-small scale. The 
length of some of the largest acerate spicules would bring 
them into the scale of the third or medium-large forms. 
Spicules of the first or second dimensions do not occur in this 
fossil. When compared with the spicules of recent species of 
Leucandra or of other genera of Leucones, as depicted in 
Heckel’s Monograph (mostly on the scale of 100 diameters), 
the fossil forms are seen to be as a rule smaller and more deli- 
cate than the recent ones. In some recent species, such as 
Leucandra Gosset, Bowbk., sp., and L. crambessa, Hack., the 
rays of the three-rayed spicules are of about the same length, 
but somewhat more robust than the fossil forms; but the 
acerate spicules in the same sponges are of unmistakably 
stouter proportions than those in the fossil, and they further 
differ in not being inflected near the point. In the fossil the 
simple acerate spicules are more numerous than the three- and 
four-rayed forms, whilst in most of the recent species of the 
genus the reverse proportions exist. 
Owing to the small size and state of preservation it is not 
practicable to ascertain the details of the canal-system suffi- 
ciently, so as to compare the fossil with its recent analogues ; 
but the evidence, so far as it goes, tends to show that there is 
the same system of irregular anastomosing canals as in recent 
Leucones. 
The significance of this discovery of fossil Leuconid sponges, 
structurally similar to the existing genus Leucandra, in strata 
of Liassic age, may be understood from the fact that hitherto 
no fossil sponge of this family has been met with, though 
detached spicules, probably belonging to sponges of the same 
group, have been detected in the Tertiary deposits of St. Erth, 
Cornwall (Quart. Journ. Geol. Soe. vol. xlii. (1886) p. 214). 
With the single exception of Protosycon punctatum, Goldf., 
sp-, from the Jurassic Limestones of Streitberg, in Franconia, 
which has been placed by v. Zittel in the Sycones family, no 
member of either of the three existing families of Calcisponges 
