68 SILICEOUS SPONGES. 



branching extremities are thickly covered with minute tubercles. The spicular 

 arms are "202 mm. in length by •067 mm. in thickness. 



The extreme variability in the form of this species is well exemplified in the 

 numerous specimens in the Museum. 



Distribution. Upper Green Sand : Warminster, Wiltshire. Red Chalk : Hun- 

 stanton, Norfolk {Prof. Seeley). Craie Chloritee: Vaches Noires, Havre. 



Hallirhoa costata, var. brevicostata, Michelin. (Plate XIV. fig. 2.) 



1847. Hallirhoa brevicostata, Michelin, Icon. Zooph. p. 127, t. 31. f. 2. 

 1878. Hallirhoa brevicostata, Zittel, Studien, II Ab. p. 79. 



Sponge in form like a partially opened mushroom, the summit depressed, the sides 

 indistinctly divided into five or six shallow lobes, the stem thick at its junction with 

 the body and gradually tapering. The only example in the collection is 39 mm. 

 in height and 75 mm. in width. This form can hardly be regarded as more than 

 a variety of H. costata. It differs from the typical forms of the species in the slight 

 degree in which it is lobed and in the thickened stem. Michelin has erroneously 

 referred Hallirhoa (Polypothecia) ag arid for mis, Benett, to this variety, but the two 

 forms are very distinct from each other. 



distribution. Upper Green Sand : Warminster, Wiltshire. 



Hallirhoa costata, var. TessOiVls, Michelin. 



1847. Hallirhoa Tessonis, Michelin, Icon. Zooph. p. 128, t. 34. f. 1. 

 1878. Hallirhoa Tessonis, Zittel, Studien, II Ab. p. 79. 



This variety only differs from the typical forms of H. costata in the gradual 

 tapering of the body of the sponge to the stem, instead of the sharp constriction 

 usually present between the base of the body and the summit of the stem. 



Distribution. Craie Chloritee : Vaches Noires, near Havre (Tesson collection). 



Hallirhoa costata, var. elevata, Ilinde. (Plate XIV. fig. 3.) 



In this variety the body of the sponge is formed by five subequal, elongated, 

 compressed lobes ; the central portion of the summit is distinctly elevated, with the 

 cloacal aperture at the top. The upper margins of the lobes are separate from each 

 other, and more compressed than the sides. The cloacal aperture is 15 mm. in 

 width. The single example is 110 mm. in height and 100 mm. in width. This 

 variety conspicuously differs from the typical lobed forms of//, costata in the elevated 

 summit and in the narrow upper margins of the lobes. 



Distribution. Upper Green Sand : Warminster. 



