212 DR. F. .T. NORTH ON [vol. lxxvi, 



from the Lias, and the Permian shells in this country are smaller 

 than either. The small size of the Permian specimens is doubtless 

 due to the unfavourable conditions under which they lived ; while 

 the large size of the Liassic species is in keeping with the fact that 

 the race was nearing extinction. 



Altbough there is every reason for believing that such is the 

 case, it is not here definitely asserted that the Permian and Liassic 

 species mentioned are homogenetic with the Carboniferous species 

 cited as examples of the two types. A discussion of the post- 

 Carboniferous forms must be deferred, until it has been possible to 

 examine the holotypes (where they still exist) or topotypas of the 

 species concerned. 



The whole series of laminose Spiriferids affords an interesting 

 example of the conditions referred to by the late Dr. Ivor Thomas 

 when discussing the Productids 1 : — 



■ ' It is becoming more and more recognized that possession of similar features 

 alone may be misleading and an insufficient criterion for the estimation of 

 species. Externally the individuals may be morphologically almost indis- 

 tinguishable, and yet belong to totally different lines of development and 

 therefore to different genera.' 



(b) Punctospirifer, gen. nov. 



Description. — Shell spiriferoid, about twice as wide as long, 

 greatest width at or near the hinge-line ; cardinal extremities 

 slightly rounded or subangular ; area moderately high and concave. 



Pedicle-valve subpyramidal, lateral slopes evenly convex, mesial 

 sinus wide and shallow; beak small, pointed, and slightly incurved. 



Brachial valve convex; mesial fold well developed, rounded, dis- 

 tinctly raised above the general level of the valve, and about as 

 wide at the anterior margin as three of the adjacent costal and the 

 furrows separating them. 



Lateral slopes of both valves ornamented by rounded costse, 

 separated by rounded furrows, about equal to them in width. 



Surface of both valves crossed by regularly-disposed imbricating 

 lamella;, which in adult specimens may be more or less obsolescent, 

 especially in the posterior portion of the valve. 



Shell-structure fibrous and strongly punctate. 



Internal structures: — Pedicle - valve. — Delthyrial! sup- 

 porting-plates slightly divergent : median septum well developed, 

 thickened at its base, and near the beak extending about two-tliirds 

 of the distance from the floor of the valve to the area ; it terminates 

 somewhat abruptly about half-way towards the anterior margin. 

 There is no marked development of an apical callosity, so that the 

 three vertical septa appear as distinct structures in sections quite 

 near to the beak. 



Brachial valve. — A low median crest bisects the muscular 

 scars ; the spiral coils are large, and have their apices directed , 



1 ' British Carboniferous Producti ' Mem. GeoL Surv. : Palaeontology, vol. i, ( 

 pt. 4, p. 253. 



