from the Middle Lias of Northamptonshire. 355 



The figures of the spicules on the accompanying Plate 

 (PI. XVII. 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 Hffickel (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 

 scale, 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 

 Hajckel'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 Gosseiy Bowbk., sp., and L. cramhessa^ Haick., 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. Ertli, 

 Cornwall (Quart. Journ. Geol. Soc. vol. xlii. (1886) p. 214). 

 With the single exception of Protosycon punctatum, Goldf.. 

 sp., from the J urassic 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 



