12 Prof, W. King on Spirifer cuspidatus. 



ing some large specimens from the neighbourhood of Galway*, 

 of which the outer layer is completely silicified, exhibiting, as 

 a consequence, merely the mineral structure (siliceous pins 

 enclosed in cylinders) characteristic of palliobranchiate shells 

 in the metamorphosed condition ; on the contrary, the inner 

 and much thicker layer consists of a greenish substance, which, 

 from effervescing on the application of acid, and being some- 

 what harder than calcite, may be considered to be arragonite 

 All the sections I have made of the latter layer display the 

 fibres more or less clearly ; but none of them exhibit the per- 

 forations at all well ; and these structures are very often alto- 

 gether absent. In their most obvious condition the perfora- 

 tions, or, speaking more precisely, their vestiges, appear as 

 dusky spots, or aggregations of granules, which have a trans- 

 lucency more or less approaching that of the fibres : both kinds 

 are ill-defined ; and the latter are occasionally larger than the 

 former. The fibres agree with those of the Redesdale speci- 

 mens in undulating and parting asunder where the traces of 

 perforations make their appearance (see figure 7). In many 

 cases little more than mere openings in the fibres are the only 

 evidences of a perforated structure ; and often, as in the Redes- 

 dale specimens, even such indications have been obliterated, 

 the fibres running on without any strongly marked deviation 

 from parallelism tt 



The histology of S^iriferina laminosa^ and the changes it has 

 undergone, bear a strong resemblance to what have been pointed 

 out in Spirifer cuspidatus^ with this difference : the perfora- 

 tions are much the smallest in the latter species, and the fibres 

 are not separated by their intrusion. But had the perforations of 

 8p. cuspidatus been of the ordinary size, there can be no doubt 

 the imperforate spaces^ and specimens^ would have exhibited 

 indications of them in the occasional opening out of their 

 fibres where they have been present. Mr. Meek has supplied 

 me with a piece of evidence which explains, as is equally 

 the case with similar evidences furnished by Nos. 1 & 2 speci- 

 mens, the absence of such traces of a perforated structure on 

 the view I have advanced. He informs me that " in some 

 instances " [the expression shows how exceptional they are] 



* The Irish specimens, which are larger than those I have examined 

 from Redesdale, have often two more obscure ribs on the terminal point 

 of each wing : the latter do not appear to have been individuals of such 

 free growth as those found in Ireland. 



t Siliceous pins occasionally occur in the fibrous inner layer ; but they 

 break through the fibres without causing them to part asunder : the latter 

 terminate abruptly against the former, as is the case with the enlarged 

 perforations in Mr. Morton's specimen of Sp. cmjndatus (see fig. 6). 



