part 2] SYBIKGOTHYRIS AND SPIMFERWA. 193 



to see whether his specimen was correctly named. His figures T 

 certainly do not indicate anything like Sowerby's holotype of 

 Spirifer distans. One of them (fig. 5) shows a threefold division 

 of the area, and a delthyrium relatively wider than that of 

 Sp. distaus, while fig. b' is obviously a Syringothyris. Neither of 

 the figures is sufficiently clear to admit of specific determination ; 

 but they most probably represent one of the Lower Avonian forms 

 of Syringothyris. 



There can, therefore, be no doubt that the possession of a non- 

 plicate fold and sinus is an essential feature of Syringothyris. So 

 •far as I am aware, the three sj)ecies enumerated in this section 

 are the only species that are supposed not to conform with this 

 rule, hut in no case does the supposition hold good. It was 

 necessary to establish this fact clearly, because it has an important 

 bearing upon the phylogeny of the genus, in that it limits the 

 stock to a group of Spirifers with a smooth fold and sinus : 

 that is, a group corresponding approximately to the 'ostiolati' 

 of Hall & Clarke ; for, although a high area in the pedicle-valve 

 may be developed in several members of the Spiriferidse (as, for 

 example, Cyrtia, Spirifer, etc.), it is only in forms with a smooth 

 fold and sinus that the feature is accompanied by the formation of 

 a syrinx-bearing transverse plate. 



In his description of Sp. distans, Davidson says that the fold on 

 the brachial valve may extend to the front margin, as in the' 

 typical form, or may soon become converted into a central rib. 2 



There is, in the Carboniferous Limestone of Derbyshire and 

 Lancashire, a well-defined form which possesses the last-mentioned 

 character, and it has usually been referred to Sp. distans. In 

 general appearance : that is, in the height and curvature of the 

 area, magnitude of the apical angle, width of the delthyrium, and 

 tumidity of the lateral slopes, it closely resembles the elongate 

 type of that species ; the peculiarity of the fold and sinus, however, 

 is so striking that the form is best regarded as a distinct species, 

 and it is described below as Spirifer plicatosulcatus, sp. nov. 



The presence of a median costa on the fold and sinus recalls 

 Sp. triangularis (Martin) ; but the two species are totally different 

 in all other respects. 



Spirifer plicatosulcatus, sp. nov. (PI. XIII, figs. 17 a 

 &17b.) 



Description. — Shell transversely trigonal, greatest width along 

 the hinge-line, cardinal extremities acutely angular. The dimen- 

 sions of the holotype are : — width along hinge-line, 04 mm. ; 

 length of brachial valve, 23 mm. ; height of area, 21 mm. 



Pedicle-valve sub-pyramidal, cardinal angles sharp, and lateral 

 slopes evenly convex. Cardinal area nearly flat in the neighbour- 

 hood of the hinge-line, but becoming increasingly convex towards 



1 De Koninck [18] pi. ii, figs. 5 & 6. 2 Davidson [4] p. 46. 



