436 MR. S. S. BTJCKMAJST ON [Aug. T 9°6, 



If it be asked why two generic names, Antinomia and Pygope, 

 should be necessary, the answer is — the evidence seems to show 

 that, used in the manner just set forth, these names designate two 

 genetic series, in which, remarkable as it may seem, the peculiar 

 perforate shape has really been developed independently twice over. 

 There are characters running through the species of each series, 

 such as the size and position of the perforation, the shape of the 

 side and the curvature or otherwise of its margin, which enable the 

 morphic equivalents of the two series to be separated. 



Nor does this fully state the case. There is actually another line 

 of these perforate Terebratulids, and this has a special character of 

 its own separating it from Pygope, though in other respects it 

 is similar. It seems, therefore, that the perforate character — or 

 rather the growth of two large lobes and their ultimate junction — 

 has actually been developed three times over, and that such 

 development took place at three closely-successive dates about the 

 end of the Jurassic and beginning of the Cretaceous Period. 



Before enlarging on this subject, it may be well to notice certain 

 other species of these perforate Terebratulids which have received 

 names. 



One species has been generally overlooked, in fact I have found 

 only a solitary reference to it. It is the Terebratula Duvali, 

 Newman, named by him in the 'Zoologist' in 1S44 \2b']. I interpret 

 this species as being a Pygope, and a near ally of T. deltoidea. 



A very remarkable species, with a large perforation, is the 

 Neocomian shell to which A. d'Orbigny gave the name T. diphyoides 

 [27]. I take as the type of his species the specimen which he 

 considered an adult form (pi. dix, figs. 6, 7, 8, & 9). Since the 

 other forms which he figures are not ontogenetic stages (as he 

 supposed), but represent the different phyletic stages in the pro- 

 gressive development of a perforate Terebratulid from a Glosso- 

 thyridoid, it is important to select one particular form as the type. 



Another species to be compared with this one, showing its 

 character of large perforation, is Terebratula janitor, Pictet [29]. 

 This name again is applied to a series of forms similar to those called 

 T. diphyoides ; that is to say, it denotes a series of phyletic stages. 

 But there is a distinction running through the series T. diphyoides 

 which separates it from the series T. janitor ; namely, that in the 

 ventral valve from the beak to the perforation there is a sulcus : — 

 that is to say, in the stage which is the morphic representative of 

 the Glossothyridoid ancestor, in the T .-diphyoides series there is a 

 sulcus in the ventral as well as in the dorsal valve 1 ; but in the 

 T. -janitor series there is not this ventral sulcus, nor is it present 

 in Antinomia. Apparently then the T. -diphyoides and the 

 T.-janitor series represent forms developed along somewhat parallel 

 but independent lines. Either the glossothyridoid ancestor of the 

 former series developed into a T.-Euthymi stage (Pl.XLI, fig. 17) — 



1 A corresponding fold within the dorsal sulcus accompanies the development 

 of the ventral sulcus. 



