Davidson — On Syringothyns. 313 



valve as in that genus, nor is it connected with the median septum 

 as in Spirifera granulifera. Nevertheless, it would seem that the 

 three structures are modifications of the same elements." 



But what is the element thus modified ? Prof. King suggested 

 that the shoe-lifter process of Cleiothyris concentrica is a modified form 

 of the ventral median plate ; but the wide separation of its points of 

 origin from the normal position of this plate seems incompatible with 

 such a conclusion; while in Syringothyris and Spirifera granulifera 

 the median plate exists independently of the apparent homologue of 

 the shoe-lifter, Mr. Billings, whose observations are generally 

 marked by extreme sagacity, regards the shoe-lifter "as an abnormal 

 form of the pseudo-deltidium that occurs in some Spirifers." This is 

 the relationship pointed out above ; and there seem to exist good 

 morphological reasons for regarding the fistuliferous arching plate of 

 Syringothyris as a modified pseudo-deltidium. But to what does the 

 latter structure appertain? In Merista, Syringothyris, and certain 

 SpirifercB its relation to the dental plates suggests that it may be an 

 out-growth of those parts. The dental plates are amongst the most 

 heteromorphous structures of the ventral valve> From a normal erect 

 position, they become approximated along the ventral margins in 

 many Spiriferce. and other genera, while in Pentamerus, Orihisina, 

 and Camaraphoria, this approximation results in complete union, and 

 in Lept<sna in the formation of the saucer-shaped process of the 

 ventral valve. They also vary excessively in longitudinal develop- 

 ment. In many Spirifene, moreover, there is an evident indication 

 of a longitudinal folding of the dental plates, producing on one side 

 or the other a longitudinal laminar process, which, under an extra- 

 ordinary development, may coalesce with some neighbouring part. 

 While, therefore, the shoe-lifter process of Merista, and still more 

 the fistuliferous diaphragm of Syringothyris, may be but modifications 

 of the false inner deltidium of Spirifera granulifera, the three 

 structures, accidental among Palliobranchs, may be but mere out- 

 growths of the essential and typical parts known as dental plates." 



Such is Professor Winchell's description of his genus Syringo- 

 thyris, and it would be very desirable if any gentlemen who 

 possess good examples of Sp. cuspidatus would cut some of them as 

 shown in Fig. 11 of our Plate, so as to see how far they may agree 

 or differ from Syringothyris. 



1 would also refer the reader to some extremely interesting obser- 

 vations upon this subject recently published by Professor J. Hall, 

 in vol. iv. pp. 252-257 of his Paleeontology of New York, and 

 from which I wiU here give the following extract. " If agaia we 

 look at the characters of Spirifera alta, an analogue or representative 

 of Spirifera cuspidata, we have many points of similarity with one or 

 more species in the rocks of the west and south-west (America) 

 which are usually referred to a higher position, The high area, and 

 the tranverse concave septum, which is not a true pseudo-deltidium, 

 allies it with Sp. textus, in which we find similar features. In Sp. alta 

 there has, probably, been an external convex pseudo-deltidium, and 

 between this and the septum closing the fissure, there has been a 



