part 2] SYBrNGOTHYHIS AND SPIBIFERWA. 187 



developed in certain species of Spirifer from both Devonian and 

 Carboniferous rocks, for instance : Sp. disjunctus J. de C. Sowerby, 

 Sp. duplicicosta Phillips, and Sp. striatus (Martin) ; and if, as un- 

 fortunately is seldom the case, a specimen is suitably preserved and 

 its internal structures are not obliterated by recrystallization, the 

 transverse plate can be seen when the beak of the ventral valve is 

 rubbed away. Text-figs, li & lj (p. 166) represent the structure 

 as observed in a specimen of Sp. duplicicosta from Park Hill 

 (Derbyshire), the sections being 3 and 6 mm. from the beak 

 respectively. In text-fig. li (p. 166) the structure is seen to be 

 essentially the same as in the apical region of Syringothyris. 



In text-fig. lj, although the relation between the transverse plate 

 and the delthy rial supporting-plates is the same as in Syringothyris 

 (see text-fig. 1 d), there is no trace of a syrinx. The extremity of 

 the plate is concave, as in Syringothyris; and therefore, in sections 

 farther from the beak than text-fig. lj, it is incomplete, being repre- 

 sented by two small portions on the inner sides of the delthyrial 

 supporting-plates. A similar transverse plate was observed by me 

 in a large specimen of Sp. striatus (Martin) from Kildare ; King 

 describes a section showing the transverse plate in this species, 1 in 

 which he supposed that there was actually a tubular canal, but 

 this has not been verified ; in any case, the structure figured by 

 King bears no resemblance to a true syrinx. 



It has already been shown that the species in which a syrinx is 

 known to exist have a smooth fold and sinus, and the examination 

 of the forms just considered proves that it is not present in species 

 in which the fold and sinus are plicated (this matter is more fully 

 discussed in § III of this paper, p. 189). The whole of the 

 evidence, therefore, points to the conclusion that it is among species 

 of the former type that the ancestors of Syringothyris must be 

 sought. 



In the Devonian rocks of North America there seems to be clear 

 evidence of the evolution of the syrinx. American palaeontologists 

 have described a number of species of Spirifer in winch the fold 

 and sinus are non-plicate, and in which a well-developed apical callo- 

 sity and transverse plate are seen : as, for example, Sp. granulosus 

 Conrad and Sp. aspen Hall, both from the Hamilton group (Middle 

 Devonian). In many specimens the plate is said to be more or less 

 thickened along the median line. In a variety of Sp. marcyi Hall 

 from the Upper Devonian of New York, described by H. S. 

 Williams, 3 there are two parallel ridges on the under surface of 

 the transverse plate, and that author regards the structure as an 

 incipient syrinx. 



If text-tig. Ig (p. 166), which is a cross-section of the beak of 

 Sp. marcyi, be compared with fig. 1 b, which illustrates the fully- 

 developed syrinx of Syringothyris, the relation between the two 

 forms is at once apparent. 



1 King [17] pi. iii, fig. 25. 



2 Williams [37] pp. 61-62 & fig. 16. 



