Gemis of Fossil HexactinelUd Sponges. 3 



questing him to make me a collection of these fossils ; and 

 from his successful search I am now in possession of some 

 forty or fiftj specimens, of which some five or six are in a 

 perfect state of preservation, while all exhibit the halfcup- 

 shape form which I had noticed previously. 



Outward Form (PI. I. figs. 1-8). — The sponge is verti- 

 cally and simply fan-shaped, compressed, single, sessile, and 

 adherent. In size it varies from 3 inclies to -|- of an inch 

 in height, from 2 inches to |- of an inch in breadth, and from 

 1 inch to I" of an inch in thickness. The object on which 

 the sponge grew is generally a small fragment of coprolite 

 (PI. 1. fig. 6, J), which in good specimens still remains 

 adherent at or near the point from which the sides of the fan 

 diverge. This point indicates, then, the '^ base" of our sponge ; 

 and it follows that the diverging sides of the fan are the 

 " lateral " edges, and the curved side which joins them, sub- 

 tending the angle at the point below, is the "distal" or 

 upper margin. The sponge is curved from side to side, the 

 lateral margins being slightly approximated, so as to make 

 the fan concave from side to side like a half-cup or hollow 

 half-cone. The concave is the " anterior " or " interior," 

 and the convex the "outer" or "posterior" surface. 



General Structure. — The sponge is composed of two obvi- 

 ous parts — a thin plate in front (PI. I. fig. 1, o), and a 

 thick protuberant mass behind (ibid. ])) ; a distinct seam 

 (s), which may be merely a line produced by the approxima- 

 tion of the skeletons of the two, or which may be deepened 

 into a shallow groove, defines these two parts from one another 

 along the lateral edges : on the posterior surface the distinc- 

 tion is manifest by the free projection of the anterior plate 

 beyond and above the posterior protuberance (PI. 1. fig. 2, o) ; 

 and in fractured specimens the distinction is seen to be con- 

 tinued within (PI. II. figs. 1, 2), the two structures, however 

 closely apposed, seldom if ever merging into one another. 



Anterior Plate. — The surface of this is even and smooth, 

 its thickness from back to front tolerably uniform, but slightly 

 increasing as it grows upwards from the base ; in a specimen 

 2-|- inches high by 2 inches broad and -f- inch thick it 

 measures \ of an inch at the summit, and at the base a little 

 less than half this amount. The ratio of the thickness of the 

 plate to the other dimensions of the fossil varies widely with 

 different specimens. 



The plate projects freely above the posterior protuberance, 

 and terminates in a broken distal edge. This is the case with 

 all my specimens. The anterior plate has been broken oiF, 

 either down to the level of the posterior mass or at a short 



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