166 



GENERA OF THE SUBORDERS ORTHOIDEA AND PENTAMEROIDEA 



Gyfiduln becomes secondarily smooth or nearly so in 

 the late Devonian. On the other hand, smooth 

 Gyfidula fseudogaleata may represent a totally differ- 

 ent stock generically from any of the others. 



The first of the known American Pentamerinae 

 are the costate Platymerella and Virgiana. They were 

 followed by smooth forms (Pentamerus, Pentamer- 

 oides, etc.), and these were later succeeded by Con- 

 chidium and allies. The strongly costate forms are 

 certainly characteristic of the high Middle and Upper 

 Silurian, especially of central United States and Alaska. 



Loss OF THE VENTRAL SEPTUM. — Many pentam- 

 erids eliminate more or less of the ventral septum. 

 Complete loss was nearly achieved by Pentamerella 

 and some species of Gyfidula, and total loss of the 

 septum is the case in Cymbidium and Holorhynchus. 

 On the other hand, Platymerella and Virgiana, earliest 

 of the Pentamerinas, had exceedingly short dorsal septa, 

 and some later stocks had longer ones. 



Development of a cruralium. — Among the 

 galeate Pentamerinae Sieberella and Barrandella alone 

 have cruralia. In the Pentameridas the genus Pentam- 

 eroides is the only one known to have this structure. 

 Conchidium with a cruralium is theoretically possible, 

 but so far none is known. 



PARALLEL TRENDS 



The subfamily, on the basis of external characters, is at 

 once separable into two divisions, Laeves and Costatae, 

 with the following arrangement in equivalent states of 

 development: 



Smooth 



Pentamerus 



Pentameroides 



Lissocaelina 



C apelliniella 



Holorhynchus 



Har-pidium, 



Costate 



Platymerella, Rhifidium 



None as yet known 



Conchidium, 



Brooksina 



Cymbidium 



None known, but to be 

 looked for in Conchid- 

 ium with abbreviated 

 septum 



We do not mean to say that these two groups repre- 

 sent two genetic lines. This is possible, but as yet we 

 do not know pentamerids well enough to state it as a 

 fact. There are too many possibilities in the way of 

 smooth stocks becoming costate and costate stocks 

 going back into the smooth state. However, our 

 scheme is interesting, even if not established. It will be 

 noticed that out of the six possibilities, four of the 

 smooth stocks have a costate genus in a corresponding 

 stage of development. 



A number of interesting parallel developments were 

 seen during this study in various groups of the pentam- 

 erids. One of these is in the development of a 

 cruralium. Camerella has a cruralium in the dorsal 

 valve but its homoeomorph, Parastrophinella, has the 

 dorsal septa separate. In Barrandella there is a cru- 

 ralium but in Clorinda, its homceomorph, the dorsal 

 plates are widely separate. The same is true of Pen- 

 tamerus and Pentameroides, and of Sieberella and 

 Gypidula. Hall and Clarke^ regarded this develop- 

 ment of a cruralium as of not more than specific 

 value and placed more reliance on the external form 

 of the shell. The trend of more recent times is, how- 

 ever, to take the opposite view, regarding the internal 

 variations as of greater importance than the exterior. 

 Accordingly we recognize the presence of a cruralium 

 as of generic value. 



SUMMARY 



In summarizing the above remarks, we can not sur- 

 vey the evolution of the pentamerids in any clearer 

 way than by showing our conceptions of the interrela- 

 tions of the various members of the Pentamerinae. 



' Pal. N. Y., vol. 8, pt. 2, p. 246. 



Family CAMERELLID^ Hall and Clarke 

 1894 



(Syn. Parastrophinina; Schuchert 1929) 



Primitive, small, usually multicostate Pentameracea 

 with the dorsal shell the deeper and with small, nar- 

 row ventral interareas; probably derived out of the 

 Syntrophiidse. All have a spondylium duplex. Dorsal 

 cardinalia tripartite, with the septal plates either dis- 

 crete or united into a cruralium supported by a median 

 septum; arm supports short. Adductor muscles at- 

 tached to the floor of the dorsal valve in front of the 

 septal plates. Delthyrium open and but rarely modi- 

 fied by narrow delthyrial marginal growths. 



Embraces the following genera: 



Camerella Billings 



? Branconia Gagel 



Parastrophinella Schuchert and Cooper 



Anastrophia Hall 



Metacamerella Reed 



The genetic relations appear to be as shown in 

 Table 18. 



