Haddon and Shackleton — A Revision of the British Aetinice. 659 



radial ridges. Disc with distinct radii ; mouth ellipsoidal, lips prominent. 

 Tentacles in two cycles of about 21 in each; length about the diameter of the disc. 



Colour. — Creamy white ; polyps with a slight pinkish tinge. 



Dimensions. — (In spirit). A. The larger specimens : height of column, 20 mm. ; 

 diameter, 4-5 mm. ; diameter of disc and tentacles, 10 mm. ; the ccenenchyme of 

 one colony measured 60 mm. by 30 mm. (PL lviii., fig. 37). B. Medium specimens: 

 height of column, 16 mm. ; diameter, 3 mm. ; average diameter of disc and 

 tentacles, 9 mm. C. Small variety : average height of column, 5 mm. ; diameter, 

 4 mm. (PI. LVIII., fig. 38). 



Locality. — West of Ireland (5—8 miles W. of the Great Skellig, Co. Kerry, 

 70-80 faths., July 13, 1886. A. C. H.). This species was also obtained by the 

 "Porcupine" in 1869. (No locality. Norman collection). 



Body-wall (PI. Lix., figs. 9, 10). — The ectoderm is continuous, and is covered by 

 a thin cuticle. It forms a thick layer, consisting of very granular columnar cells, 

 which stain deeply, and of numerous nematocysts which do not stain. The 

 nematocysts in this species are scattered throughout the ectoderm in a fairly 

 uniform manner. Incrustations consisting of spicules, grains of sand, and 

 foraminif era may be found scattered at intervals through the mesogloea, but in our 

 specimens of the larger variety these are very rare. Beneath the incrustations 

 lies a well-developed encircling sinus. It is frequently broken by strands of 

 mesogloea, and is connected with the peripheral ectoderm by numerous branching 

 and anastomosing canals, very similar to those we find in P. anguicoma. The 

 encircling sinus is connected with the endoderm by the fibrils or canalaculi of the 

 mesogloea, which are numerous and very distinct in our sections. The endoderm 

 is not of uniform thickness as in P. angiiieoma, but becomes very thick in the 

 centre of each endocoele and ectocoele, thus forming a longitudinal ridge between 

 every two mesenteries. The diffuse endodermal muscular layer is well developed. 



Sphincter muscle (PL lx., fig. 8). — The diffuse endodermal sphincter is well 

 developed, but very simple in character, the mesogloea being raised into distinct 

 but unbranched plaitings. In some sections some of these plaits appear to unite 

 so as to enclose part of the muscle entirely in the mesogloea, but we are uncertain 

 whether this appearance is not due to the direction in which the sections are cut. 



Disc and Tentacles. — There is little worthy of note in the structure of the disc 

 and tentacles. The ectodermal muscular layer is well developed. 



(Esophagus. — The ectoderm of the oesophagus is thrown into deep folds, into 

 which the mesogloea also enters. There is a deep, well-marked groove, and the 

 mesoglcea is here very much thickened. 



Mesenteries. — The arrangement of the mesenteries is macrocnemic. The 

 imperfect mesenteries are well developed, and extend into the body-cavity nearly 

 half-way between the body-wall and the oesophagus. The ectoderm of the 



5 A2 



