4 Clonings — Morphogenesis of Platystrophia. 



Dorsal valve. — As in the ventral valve the posterior half is 

 smooth, with faint concentric lines. The anterior half is occu- 

 pied by eight plications. The depression (sulcus) between the 

 two inner ones is deeper and broader than between the others, 

 giving the effect of a median sinus corresponding to the large 

 median plication or fold of the ventral valve. The convexity 

 of the dorsal valve is nearly as great as that of the ventral. 



Comparison with nepionic stages of other Orthidce. — At the 

 stage represented by the youngest specimens of Platystro- 

 phia (l mm broad), Dalmanella (testuclinaria, elegantula) and 

 Rhipidomella hybrida* have twelve plications on each valve, 

 which arise very close to the apex. The valves are convex, 

 though the dorsal usually has a sinus (sometimes conspicuous) 

 toward the front. The areas are high and well developed, not, 

 however, extending to the cardinal extremities. The ventral 

 area is noticeably higher than the dorsal. The delthyria in both 

 valves are large and open. 



Bilobites at this stage has nine plications on the ventral 

 valve, one of which lies at the bottom of a shallow median 

 sinus. These originate, as in Platystrophia, about half-way 

 from the beaks to the front margin. The areas are high and 

 the delthyria large and open. The relative convexity of the 

 two valves is almost exactly the same as in Platystrophia.^ 



Hebertella at l*4 mm ? the youngest stage yet seen, has eighteen 

 plications which originate very near the beak. The valves are 

 both convex, the ventral being considerably higher. The areas 

 are high and about perpendicular to the plane of separation of 

 the valves. 



All the young Orthidce seen, with the exception of Bilobites, 

 have noticeably transverse shells :J but in all other respects, 

 except the index, the young Bilobites most closely resembles the 

 nepionic Platystrophia. 



Comparison with adult stages of other Orthidw. — There are 

 no adult Orthidce in the Ordovician with which it is possible 

 to compare the nepionic shell of Platystrophia.^ Wysogorski 

 has suggested the derivation of the genus from the Orthis calli- 

 gramma group. | His views are based entirely upon adult char- 

 acters of both groups. He says (loc. cit.) u In both groups the 



*For the development of Dalmanella elegantula and Rhipidomella hybrida 

 see Beecher and Clarke, Mem. N.Y. State Museum, i, 1889, pp. 13-18. 



f For the development of Bilobites, see Beecher, this Jour, xlii, July, 1891, 

 pp. 51-56. 



tThe index of Dalmanella is 1*33 or more, and of Hebertella 1*43. That 

 of Bilobites is only 1*13 (Beecher's figures). 



§ Orthis kqnvorthi Davidson, from the Llandeilo — Middle Caradoc of Eng- 

 land, comes the nearest, and is probably an almost lineal descendant of Orthis 

 lenticularis of the Upper Cambrian. 



|| Zur Entwicklungsgeschichte der Brachiopodenfamilie der Orthiden im 

 ostbaltischen Silur. Sonder-Abdruck aus der Zeitschrift d. Deut. Geol. Ges. 

 Jahrg., 1900, Heft 2, p. 14. 



