GENUS SPIRIFERA. 255 



" It should be observed, in this connection, that the dental plates, both in 

 Spirifera alt a and S. textus, reach to the bottom of the cavity, and 

 partially surround the muscular impression, which is precisely like that of 

 ordinary Spirifers. 



" In these remarks, I have not intended to express an opinion of the 

 generic value of certain characters ; but merely to show, as it appears to 

 me, a gradual or successive development in certain parts, which finally 

 becomes so wide a departure from the characters of typical forms of Spi- 

 rifera as to deserve especial attention. Nor can we deny that this 

 progressive development of the septum and its modification keeps pace, and 

 corresponds, with the geological succession ; reaching its extreme state, so 

 far as now known, in the Carboniferous period, where it is connected with 

 a punctate texture of the shell,"* 



Note. — The Spirifera alta referred to in the preceding pages is an 

 analogue of the European carboniferous Spirifera cuspidata, having a 

 similar elevated area which is usually " slightly inclining forward or nearly 

 rectangular to the general plane of the dorsal valve. The fissure is high 

 and narrow, and is closed for two-thirds of its length from the apex by a 

 concave septum which is entirely independent of the pseudo-deltidium." 

 On page 249 of Vol. iv. Pal. N. Y., I have made the following remarks 

 under the description of the species : 



" This species is known to me only in the condition of casts of the 

 interior, and its usual appearance is illustrated in the figures on Plate xliii. 

 Its general aspect is much like that of the European Spirifera cuspidata^ 

 Martin ; but there are important differences by which it may be dis- 

 tinguished : these are, the plications on the mesial fold, tiie larger area of 

 the dorsal valve, and the shorter extension and greater divergence of the 

 dental lamellae by the sides of the muscular impression, ^ome of these 

 characters, I conceive, are not likely to change to those shown by S. cus- 

 pidata. In the concave septum closing two-thirds of the fissure from above, 

 it resembles that species as described by Prof. M'Coy, who mentions the 

 presence of a 'deep-seated pseudo-deltidium. 't In one of the figures 

 given by Mr. Davidson and referred with doubt to this species, I the cast 

 shows a tubular perforation in the filling of the fissure, and a gutta percha 

 impression from the same shows the mark of a foramen; but there is no 

 positive evidence of a septum which is so conspicuous in our specimens, 

 and which I suppose to be the feature characterized by Prof. M'Coy as a 

 deep-seated pseudo-deltidium. In our species, I have not been able to dis- 

 cover any corresponding perforation ; the only indication of this being the 

 semicylindrical impression along the centre of the fissure (in the cast), 

 showing a callosity of the septum behind the exterior wall. 



* See note on page 256. 



f * * * *; triangular opening very large, often displaying the internal deep- 

 seated pseudo-deltidium (without perforation, leaving the only opening to the shell at its 

 base); * * *. M'Coy, British Palaeozoic Fossils, p. 42fi. 



X Monograph of British Carboniferous Brachiopoda, Plate ix, f.l & 1 a. 



