314? PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



Plate XIII. 

 Fossil remains q/" Pteropodous Shells yrom the North Welsh Silurian Rocks. 



Several species belonging to the genus Creseis had been confounded with the 

 genus Orthoceratites, until Prof. E. Forbes pointed out their true character : these 

 fossils are of great assistance in classifying the middle part of the Silurian system in 

 North Wales, as they are found in rocks otherwise nearly bare of organic remains. 

 The author found the four following spe jies in the Wenlock rocks of Denbighshire 

 and Merionethshire : — 



Creseis primteva, E. Forbes, Quarterly Geological Journal, vol. i. p. 146. 

 Plate XIII. fig. 2. — Sheath long and regularly tapering, smooth, with one 

 (or more ?) longitudinal grooves. Length two to eight inches ; width of 

 the aperture one-twelfth of the length. Very common in the Wenlock 

 rocks of Denbighshire and Merionethshire. 



Creseis ventricosa, n.s., Plate XIII. fig. 3. — Sheath straight at the back, slightly 

 ventricose in front, tapering to a point ; nearly smooth with faint sloping 

 lines of growth, and a strong longitudinal furrow. Length two and a haltf 

 inches ; mdth of the aperture half an inch. In Wenlock flagstone, in the 

 Tyn-y-ffridd quarry between Cerrig-y-Druidion and Ruthin. 



Creseis obfusa, Plate XIII. fig. 4. — Sheath short and conical, sides slightly 

 curved towards a blunt point, smooth ? with a longitudinal furrow. Length 

 two inches ; width of the aperture half an inch. Very common in the 

 Wenlock rocks of Denbighshire and Merionethshire. 



Creseis gracilUma, Plate XIII. fig. 5. — Sheath very long, straight and slender, 

 tapering gently to a point. There is only one broken specimen of this 

 species, four inches long, one-eighth of an inch wide at the upper end. In 

 Wenlock flagstone in Tyn-y-tfridd quarry between Cerrig-y-Druidiou and 

 Ruthin. 



The following species belonging to the same family is added for the sake of 

 illustration, although found in another district : — 



Theca Forbesii, Plate XIII. fig. 1. — Sheath nearly flat behind, rounded in front, 

 conical and tapering to a point ; aperture an obtuse-angled triangle, with 

 the angles rounded off ; surface covered with fine striae, arched parallel to 

 the curved edges of the aperture. Length half an inch to one inch ; width 

 at the aperture one-third of its length, dommon in the middle part of the 

 Ludlow rocks at Underbarrow near Kendal. This species is closely allied 

 to T. lanceolata of Morris, found in palaeozoic rocks in New South Wales. 

 a and b, internal casts ; the figm-e above a is a section to show the form of 

 the aperture ; c and d, exterior of the sheath. The author names it after 

 Professor E. Forbes, who first pointed out the class of animals to which 

 these fossils belong. 



