Yol. 62.] BEACHIOPOD HOMCEOMOEPRY. 437 



the stage of ventral sulcus added — before the lobate stage was 

 commenced ; or the Terebratida-janitor-seTies, if it had such an 

 ancestor, has managed to lose all trace of this ventral-sulcus stage 

 which the T. diphyoides retains so markedly. 



In order to distinguish the diphyoides-series of genetic forms, I 

 propose to use de Haan's MS. name Pygites. I do this, because 

 T. janitor belongs to the same series as T. deltoidea, that is to say 

 it is a Pygope. In both these species there is no dorsal sulcus in 

 the preperforate stage ; and yet their perforation is larger and more 

 anteriorly situated than in Antinomia. 



Thus there would appear to have been, from a Glossothyridoid 

 form, three independent lines of development into bifidate and 

 perforate stages (see p. 438). 



The forms of the different stages in the three developmental lines 

 are thus distinguishable : — 



Glossothyridoid Stage. 



Antinomia. Transverse shape. 



Pygope. Very transverse. 



Pygites. Very transverse, develops dorsal fold and ventral sulcus. 



Bifidate Stage. 



Antinomia. Division of lobes carried close to umbo. Lobes long, 

 not very wide. Side flattened, and margin much curved. 



Pygope. Division of lobes not so near umbo. Lobes wide, not 

 very long. Side more or less acute ; margin straight. 



Pygites. Like Pygope, but with an extra fold and furrow in the 

 preperforate stage. 



Perforate Stage. 



Antinomia. Perforation small and close to umbo. Sides flattened ; 



margin much curved. 

 Pygope. Perforation large, and somewhat distant from umbo. 



Sides subacute ; margin straight. 

 Pygites. Like Pygope, but with extra fold and furrow. 



As to the fold and furrow, Pictet says [29, p. 164] : — 



'It [T. janitor B.l.=Pygope\ is distinguished very clearly [from T. diphyoides 

 &X= Pygites] by ... . its dorsal [ventral] ridge being completely destitute of 

 furrows, and by the absence of a median ridge on the base of the small valve. 

 I bave bad in ray bands at least 60 examples, which, compared with my 

 200 specimens of diphyoides, bave never furnished any case of transition or of 

 doubt.' 



Pictet may also be consulted with advantage as to the general 

 features which separate ' the species with small perforation ' 

 [Antinomia] from the janitor-diphyoides forms. The development of 

 these three genetic series through these three stages may be 

 followed in PL XLI, and may be stated somewhat as follows : — 



Antinomia is descended from a rather transverse Glossothyris- 

 like form, which took on compression of the lateral margin 



