462 Rev. N. Glass — Spiral of Sjoirifera glabra. 



in Davidson's Carboniferous Supplement, pi. xxxii. fig. 4. This 

 specimen shows the spirals nearly filling the shell, and it was 

 regarded by Davidson as completely disposing of McCoy's attempt 

 to erect Sp. glabra into a new genus. 



But in June, 1890, I obtained, during a visit to Castleton, a larger 

 specimen of Sp. glabra than that just re- 

 ferred to, and of a somewhat different shape, 

 being longer in proportion to its width. A 

 portion of one side of this specimen had been 

 broken away, but in the other portion I 

 succeeded in working out one of the spirals 

 in situ, and a small portion of the other 

 spiral, showing, however, its attachment and 

 direction. The shape of the spiral in the 

 new specimen, and the space which it oc- 

 cupies in the shell, is shown in the accom- 



Spirif era glabra, Martin, panying sketch. 



Half natural size. The spiral commences with two or three 



very large coils which extend nearly the 

 whole length of the interior of the dorsal valve. These large coils 

 are succeeded by others of about half the diameter, which continue 

 with hardly any diminution of size nearly to the end of the spiral. 

 That part of the spiral which is formed by the smaller coils projects 

 outwards towards the lateral margins of the shell from the posterior 

 half of the larger coils — the posterior border of the whole of the spiral 

 being thus nearly straight, and lying close under the hinge-line of 

 the shell. This shape of the spiral, so far as its anterior border is 

 concerned, is quite unique. Out of the thousands of spirals I have 

 seen during my researches amongst British and foreign specimens, 

 I have not met with one of the same shape. It has been abundantly 

 proved by recent investigation in the fossil Brachiopoda that in the 

 various specimens belonging to the same species there is consider- 

 able modification as to the shape of the spirals, and tbat this modifi- 

 cation seems to be governed in the majority of instances by the 

 variation in the size and shape of the shell (see the descriptions and 

 figures of my preparations of Spirifera striata and Atrypa reticularis 

 in Davidson's Carboniferous and Silurian Supplements) — so that 

 there is nothing abnormal in the occurrence of differently shaped 

 spirals in a newly developed specimen of Sp. glabra. 



This larger specimen of Sp. glabra which I have now developed 

 would have greatly interested Dr. Davidson because showing how 

 the mistake of McCoy might naturally have arisen. As the first 

 two or three coils of each spiral are prolonged nearly to the anterior 

 margin of the shell, the spirals cannot properly be described as 

 being " so small as only to occupy the rostral half of the shell " ; 

 whilst on the other hand it is evident that through the greater part 

 of each spiral being close to the posterior border of the dorsal valve, 

 there remains a large portion of space in the shell which the spirals 

 do not occupy. If, therefore, in the specimen seen by McCoy, the 

 prolongation of the first two or three coils of each spiral towards 



