Cumings — Morphogenesis of Platystropkia. 39 



with P. costata. The two are, however, easily distinguished 

 and, besides, have a different range.* P. costata is, moreover, 

 derived from P. lynx and not from P. laticosta, as is the case 

 with the other form. 



Platystropkia costata presents well characterized gerontic 

 stages (fig. 16, B). These are marked by extreme gibbosity 

 and great breadth anterior to the hinge-line so that the cardi- 

 nal angle may be as great as 113°. Where the secondary 

 plications of the sinus and fold are faintly marked in ephebic 

 stages, they are almost sore to become obsolete in gerontic 

 stages. The thickening of the shell does not become pro- 

 nounced as in lynx and acutilirata. 



The costata and laticosta types seem to have been produced 

 simultaneously from the same pauciplicate lynx stock. The 

 former did not survive the Middle Ordovician either in this 

 country or in Europe ;f but from it sprang the only genus, so 

 far as at present known, that can be traced to the Platystropkia 

 group as a radical stock. From a study of the young of 

 Bilobites, BeecherJ concluded that it was probably derived 

 from the Platystropkia group. This conclusion has since 

 been called in question by Wysogorski,§ who maintains that 

 the punctate character of Bilobites removes it from the 

 "Orthis group," in which he places Platystropkia, and relates 

 it to the " Dalmanella group." It is well known, however, that 

 punctate and impunctate shells occur in a variety of diverse 

 groups of Brachiopods, and that the early members of a group 

 may be impunctate, w T hile the later members are punctate. [ 



The resemblance of the early nepionic stages of Bilobites 

 to the nepionic stages of Platystropkia has already been 

 pointed out. The similarity between a late nepionic stage 

 of Bilobites and the adult Platystropkia costata amounts 



* Platystrophia cypha James is doubtless this form of P. laticosta. He 

 mentions the extremely high fold and one strong plication of the sinus. The 

 writer has never seen as many plications as he records (22 to 26), but doubtless 

 in the upper part of its range (his specimens are from Warren Co., Ohio), 

 the form developed a larger number of plications. See James, Cin. Quar. 

 Jour. Sci., i, 1874, p. 20. 



f See Wysogorski, Entwicklungsgeschichte, p. 15. 



\ This Jour., vol. xlii, July, 1891, p. 54. 



§ Entwicklungsgeschichte, p. 9, footnote 1. 



|| The early Spirifers are impunctate ; but in the Ostiolati S. plenus is 

 punctate. Among the derived genera several are punctate, viz., Syrin- 

 gothyris, Cyrtina, Spiriferina. Among the Rhynchonellidce all the earlier 

 forms are impunctate, but Rhynchopora, of the Carboniferous is punctate, 

 though in all other respects it agrees with the earlier Rhynchonellids. 

 Among the Athyridce, Hindella, Ccelospira, etc., of the Silurian are im- 

 punctate, while Eumelria, Hustedid, etc. of the Carboniferous, are strongly 

 punctate ; Rhynchospira is sparsely punctate. The Atrypidce are impunc- 

 tate. Among the Terebratulacea, nearly all are conspicuously punctate, but 

 in the early genera, Rensselceria and Amphigenia, the punctation is incom- 

 plete. See Hall & Clarke, Pal. N. Y., viii, pt. 2, 1893. 



In Hebertella the early species are impunctate or superficially punctate : 

 the upper Richmond form is more clear ly punctate. 



