PLOCOSCTPHIA. 133 



the larger rays, can be occasionally seen, so that there is great probability that a 

 continuous membrane was originally present. 



Distribution. Silurian strata of Upper Ludlow age : Wetherlawlinn, Pentland 

 Hills, near Edinburgh. The specimens belong to 'the collection of the Geological 

 Survey of Scotland, and were discovered by Mr. James Bennie. 



Family MEANDROSPONGID^, Zittel. 



Genus PLOCOSCYPHIA, Reuss, 1846. 



Plocosctphia fenestrata, T. Smith, sp. (Plate XXVIII. figs. 4, 4 a, 4 5, 4 c.) 



1848. Brachiolites fenestratus, Toulm. Smith, ,\iin. &Mag. Nat. Hist. 2ndser. vol. i. p. 367, 

 1. 16. f. 3. 



Sponges subcylindrical, hemispherical, or irregular in form, oftentimes growing in 

 close contact with each other, so as to form masses of considerable size. The outer 

 surface is composed of numerous anastomosing, subcylindrical, open tubes, from 4 to 

 8 mm. in diameter. The central portion of the sponge, as seen in a transverse section, 

 appears to consist of open anastomosing folds of the sponge-wall, which, when simple, 

 are about 2 mm. in thickness ; but the folds sometimes coalesce laterally, and are 

 consequently of increased thickness. 



The spicular structure of the interior of the wall-plaits is generally regular ; the 

 spicules form a rectangular oblong mesh with elliptical interspaces. The distance 

 between the nodes varies from '3 to "4 mm. The spicular arms are robust, smooth, 

 or with blunt spines, and about -08 mm. in thickness. The dermal layer is composed 

 of an irregular spicular network, with minute circular or ovate pores ; the nodes of 

 the spicules are solid, whilst those of the interior of the wall are octahedral or 

 ] antern-like. 



This is a well-marked species, and readily recognizable by the size and disposition 

 of the tubes of the exterior surface. In the numerous specimens in the Museum 

 from T. Smith's collection, and labelled by him, I have not found one which corre- 

 sponds with the figure which he has given of this form. This figure appears to me 

 to be diagrammatic merely, and altogether misleading. 



Distribution. Upper Green Sand : Dover and Folkestone. Chalk Marl : Ventnor, 

 Isle of Wight ; Norton Bavant, Wilts. 



Plocosctphia labeosa, Toulm. Smith, sp. (Plate XXIX. fig. 2.) 



18-18. Brachiolites labrosus, T. Smith, Ami. & Mag. Nat. Hist. 2ud ser. ^hjI. i. p. 368, 



t. 16. f. 4. 

 1878. Antrispongia dilabyrintfdca, Queust. Petref. Bd. 5, p. 474, t. 137. f. 24. 



