454 ME. S. S. BITCKMAN ON [Aug. IQ06, 



Subperforate Stage. 



Fig. 15. Pygope solidescens, 110m. nov., after Zeuschner. — Dorsal valve, showing 

 a slight depression and a trace of the line of junction of the lobes. 

 From the southern slopes of the Tatra. (p. 447.) 



Imperforate. 



16. Pygope euganeensis (Pictet), after his figure, pi. xxxiv, fig. 5. — From 



the Neocomian. (p. 448.; 



A morphic equivalent. 



17. Terebratula Euthymi, Pictet, after his figures in 'Mel. Pal.' livr. ii, 



pi. xxv, figs. 5 a, b, d. — This species is the morphic equivalent of the 

 dorsally plicate, ventrally sulcate Grlossothyridoid, through which 

 Pygites must have come ; but it is too narrow and too globose to be 

 regarded as a true ancestor of the thin, latangulate bifidate Pygites. 



a, dorsal valve — the middle line indicating the fold in the sulcus ; 



b, ventral v r alve, showing the sulcus in the ridge ; c, anterior view, 

 showing the above characters. From the Piphyoides-Jjiuiestone of 

 Berrias. (p. 436.) 



Figs. 18-22. Pygites. 

 Bifidate. 



18. Pygites sp., after Pictet, pi. xxiv, figs. 15 & 16.— a, Young example, 



dorsal valve showing the median ridge ; b, mature specimen, ventral 

 valve showing the sulcus, (p. 449.) 



19. Pygites dijohyoides, d'Orb., the lectotype, after d'Orbigny.— a, Dorsal 



valve, with median ridge in the preperforate stage ; b, lateral view, 

 showing straight margin ; c, portion of ventral valve, (p. 449.) 



20. Pygites sp., after Pictet, pi. xxiv, fig. 12. — Dorsal valve, showing 



the sulcus. Morphic equivalent of Antinomia, cor (fig. 6). but with a 

 great difference in the size of the perforation, (p. 449.) 



21. Pygites sp., after Pictet, pi. xxiii, fig. 2. — Dorsal valve, showing the 



median ridge. A narrow form. (j3. 449.) 



22. Pygites sp., after Pictet, pi. xxiii, fig. 1. Dorsal valve. This is a 



dwarf form, and is perhaps the final expression of Pygites. (p. 449.) 

 The forms (figs. 19-22) may be, unlike Antinomia (figs. 5-7), in direct 

 genetic order of development ; for there is a rounding-off of the 

 anterior angles, and the relative width does not constantly increase ; 

 consequently, by tachygenesis, a narrower and narrower form would 

 be produced. 



Liassic Species. 

 Grlossothyridoid Stage. 



23. Terebratida Aspasia, Meneghini = Pygope Aspasia, auct., Glossothyris 



Aspasia, auct. ; after Zittel [40], pi. xiv, fig. 1. — a, Dorsal valve, 

 showing the sulcus and the sharply-reflected median linguiform lobe, 

 seen more clearly in b, anterior view. (p. 438.) 



This form is not only a morphic equivalent, it is also really a 

 homoeotnorph of Antinomia planulata (fig. 1). From the Middle 

 Lias (Pliensbachian), Cagli. 



Imperforate. 



24. Terebratula erbaensis, Suess-Pictet l = T. diphya, Suess^T. erbaensis, 



Suess MS. in Pictet =Pygope erbaensis, auct. After Pictet, pi. xxxiii, 

 fig. 8 a. 



This form may be called a homceomorph of Antinomia and Pygope, 

 imperforate species : whether it be a morphic equivalent, whether, 

 that is, it has passed through a perforate stage and is connected with 

 Terebratida Aspasia (fig. 23), is the interesting point. (See p. 439.) 



1 See p. 449, note. 



