438 MR. S. S. BTJCJOJAN OK [Aug. 1906, 



(glossothyridoid stage). Next, it developed the two long side-lobes 

 (bifidate stage, T.-sima stage). The space between the lobes was deep, 

 carried near to the umbo, but the lobes were not parted so very widely. 

 When, later, the lobes fused (perforate stage) there was produced 

 a series of forms with a small perforation fairly close to the umbo. 



On the other hand, Pygope arose from a considerably transverse 

 Glossothyridoid. This developed broad side-lobes, with a relatively- 

 broad, not deep, parting (bifidate stage) ; but it did not carry the 

 division so near to the umbo as Antinomia. Consequently, when 

 coalescence of the lobes took place, the resulting perforation was 

 larger, and farther from the umbo than in Antinomia. Then the 

 T.-diphyoides series {Pygites) arose from a transverse Glossothyridoid, 

 like Pygope did; but before attaining the bifidate stage this 

 Glossothyridoid developed further — it began plicating. It raised a 

 fold in the dorsal sulcus, and made a furrow in the ridge of the 

 ventral valve. It thus produced a plicate form analogous to 

 Terebratula Euthymi, Pictet, only- it was much more transverse. 

 Then it carried out the development of the bifidate and perforate 

 stages parallel to those of Antinomia and Pygope ; but it kept, like 

 Pygope, a relatively-large perforation. Both Pygites and Pygo}>e are 

 distinguished from Antinomia by lacking the flattened sides and the 

 curved side-margin. 



In estimating the size of the perforation, morphic equivalents 

 should always be compared. 



The development of the diphyoids from glossothyridoid to bifidate, 

 to perforate stages is very obvious ; but what succeeds to the 

 perforate stage ? The question seems to be answered by a 

 remarkable species figured by Zeuschner, for which I propose a 

 name, Pygope solidescens (see p. 447). In this species the perforation 

 is closed* — there remains, as it were, only the scar of the wound ; 

 while the line of junction of the coalesced lobes seems to have 

 completely disappeared. 



This suggests, extraordinary as it seems, that such species as 

 T. pileus, Brug. ( = T. triangulus, Val.), T. euganeensis, Pictet — in 

 fact, the imperforate forms — are the further development ; that they 

 have actually lost all trace of the perforation and of the line of 

 coalescence, and show no trace of ever having been perforate 

 Terebratulids. What is more remarkable, and shows the cycle in 

 which development so often moves, T. pileus is actually again 

 sulcatingthe dorsal valve, and recommencing a glossothyridoid stage. 



I must confess that I was quite unprepared for the idea that 

 imperforate T. pileus could have been developed from perforate 

 forms — not until I saw the Pygope solidescens. The discovery of 

 that species I owe to the chance sending of a volume from the 

 Society's Library, and therefore tender my best thanks to our 

 courteous Assistant-Librarian, Mr. W. R. Jones. 



In view of this result, an interesting surmise suggests itself. 

 There is in the Middle Lias a Glossothyridoid (T. Aspasia,M.eneghim) 

 (PI. XLI, fig. 23), with sulcate dorsal valve, a form just analogous to 



