part 2] SYliJiXGOTHYRlS AND SPIBIFEEINA. ' 223 



times, and reached its acme early in the Carboniferous Period. 

 Our knowledge of the pre- Carboniferous history of the genus is, 

 up to the present, derived entirely from its North American repre- 

 sentatives. 



5. In all known species of Syringothyris the fold in the brachial 

 valve and the sinus in the pedicle-valve are smooth. Species such 

 as Spirifer distans, in which the fold and sinus are plicated, do not 

 possess a syrinx, and have been incorrectly referred to Syringo- 

 thyris. The genus Syringopleura, founded to include shells with 

 the internal characters of Syringothyris, accompanied by a plicated 

 fold and sinus, is not valid, since the type-species S. randalli was 

 imperfectly described, and the syntypes include two distinct forms : 

 of these, those in which the fold and sinus were plicated do not 

 possess the internal characters of Syringothyris. 



(>. The form referred to by M'Coy as Spirifera laminosa, which 

 has also been regarded as belonging to Gyrtia, Spiriferina, and 

 Syringothyris, is here assigned to a new genus, Tylothyris, since 

 it has not the punctate shell-structure of Spiriferina, nor the 

 internal characters of Syringothyris. The essential characters of 

 Tylothyris are : a laminose surface-ornament, impunctate shell- 

 substance, a ventral median septum, and a peculiarly developed 

 apical callosity. The genus is represented in the Lower Avonian 

 by mutations of T. laminosa (M'Coy), and in the Upper Avonian 

 by subspecies of T. subconica (Martin). 



7. Syringothyris and Spiriferina are in no way related, either 

 morphologically or phylogenetically. 



8. The small Carboniferous shells with a punctate shell-structure, 

 laminose surface-ornament, and a ventral median septum, which 

 have hitherto been referred to Spiriferina, include two types 

 characterized by external differences and by slight variations in the 

 jugum. Of these two types, one, in which there are numerous 

 costae and a relatively-large rounded fold and sinus, is relegated to 

 a new genus, Pitnctospirifer ; while the members of the other 

 type, which include shells with a few large angular costae but little 

 smaller than the angular fold and sinus, are, for the present at least, 

 retained in Spiriferina, although the type-species of that genus is a 

 Liassic form. Both of these types belong to the family Suessiidse 

 which is represented also in Permian and Triassic rocks. The 

 subdivision here suggested for the Carboniferous forms will, it is 

 believed, prove to be applicable to the later species also. The 

 investigation of these forms, and of the relation, if any, which 

 exists between them and the Devonian and Silurian species which 

 resemble them in all but the punctate shell-structure, is not yet 

 completed. 



