GENERA OF THE SUBORDER PENTAMEROIDEA 



165 



divisible into four distinct units termed by Leidhold:* 

 (1) The inner crural plate, (2) the outer crural 

 plate, (3) the crural band or border, (4) the septal 

 plate. We prefer to term the first two of these parts 

 simply the outer and inner plates, since we do not feel 

 that it is at present certain that they are the homologues 

 of the crura such as occur in the Rhynchonellacea and 

 Terebratulacea. 



1. The inner plates are curved, convex ventrally, 

 and unite with the outer plates and the wall of the 

 valve to form a prominent umbonal chamber. The 

 sockets into which the ventral teeth fit are notches 

 in the inner plates where they unite with the wall 

 of the valve. The designation "inner plates" is not 

 entirely apt, since, when the valves are seen in ventral 

 view, the plates are actually the outermost part of the 

 cardinalia. Leidhold evidently coined his expressions 

 from a specimen that had been split longitudinally 

 (this at least is the way he figures the structures). 

 Thus seen from the side, the plates are innermost, being 

 next to the spondylium. 



2. The outer plates are rather thin and low, and 

 unite with the dorsal edge of the inner plates and are 

 separated from the septal or supporting plates by (3) a 

 band or longitudinal thickening, the "Cruralleiste" of 

 Leidhold. 



4. The septal or supporting plates unite with the 

 floor of the valve or with each other if a cruralium is 

 present, and they support the other structures. They 

 are commonly rather long and may be the best devel- 

 of)ed of the tripartite cardinalia. 



Further discussion is necessary regarding the Crural- 

 leiste or base of the brachial support as we prefer to 

 term it. This band is prolonged into a free process 

 that commonly extends to the front end of the ventral 

 spondylium. It is this elongate, free process that 

 undoubtedly was the support of the lophophore. The 

 Cruralleiste represents the growth path of this brachial 

 support or rather the remnant of the brachial support 

 which has been enclosed and encroached upon during 

 the forward growth of the inner and outer plates. 

 These brachial processes are rather stout in the Gypidu- 

 linx but in some members of the Pentamerina are long 

 and slender. In Brooksina, with its strongly arched 

 valve, the processes are exceedingly long. 



The cardinalia of the Pentamerida; as herein de- 

 scribed are absolutely distinctive, nothing like them 

 being known elsewhere. There is a slight difference 

 between the septa of the Gypidulina: and the Pentam- 

 erinje. That of the former is commonly bowed or 

 convex outward, especially where the inner and outer 

 plates unite with the base of the brachial support. This 

 is notably true of Gypidula, Sieberella, and Pentamer- 

 ella. In the Pentamerinae, on the other hand, the 

 plates are generally higher and consequently not 

 notably bowed. 



* Abhandl. preuss. geol. Landesanst., n. ser., Heft 109, 

 1928, pp. 51-53. 



GENERIC AND EVOLUTIONARY 

 TRENDS 



The following discussion points out the more im- 

 portant generic and evolutionary trends among the 

 pentamerids observed by us, and indicates as well what 

 characters we regard as of most import in defining the 

 genera. 



Loss OF INTERAREAS AND DEVELOPMENT OF A 



PLANAREA. — In all of the pentamerids the inherited 

 interareas are reduced to mere remnants and in many 

 of the genera they have totally disappeared. But along 

 with their vanishing comes the development in a few 

 stocks of planareas or so-called false areas, plane (or 

 nearly so) surfaces developed on either side of the 

 delthyrium and largest in Brooksina. This develop- 

 ment of planareas is of little taxonomic significance 

 so far as present knowledge goes. The great reduc- 

 tion of the interareas is of course characteristic of the 

 whole superfamily. 



RosTRATiON. — Elongation of the beaks is best 

 developed in Conchtdium, Harfidium, and Lissoccelina, 

 and along with this extreme rostration has come a 

 great arching of the ventral beaks over the dorsal ones. 



Reversion of normal convexity. — This is a 

 common tendency among many brachiopod stocks and 

 the cause of it is not fully understood. In Strofhomena 

 of the Strophomenacea it has been correlated by Sarde- 

 son" with a pendant growth habit, but this gravity 

 causation does not appear to be responsible for the 

 reversion in Brooksina and Cafelliniella. 



Reversion of fold and sulcus is another common 

 feature, and one that is not uncommonly attended by 

 rather profound alterations of the interior, and yet 

 Pentamerella and Sieberella are structurally alike in- 

 ternally though the fold and sulcus are reversed. In 

 Virgiana, Twenhofel' says that the reversion of fold 

 and sulcus takes place during growth, hence he regards 

 it as a generic character, but to us this is certainly not 

 so important as the internal features. 



Trilobation. — The flattened pentamerids such as 

 Pentamerus and Rhifidium developed pronounced tri- 

 lobation, which may be tied up with their incurrent 

 and excurrent canals as explained in the Orthidx. 

 This trilobation is also exhibited in the galeate pentam- 

 erids and the Camerellida by the development of a 

 fold and sulcus. 



Ornamentation. — It appears clear that the first 

 of the known Camercllid<-c were practically smooth 

 during early growth and that later the stock became 

 more and more costate. Among the Gypidulina; the 

 first members to appear, Clorinda and Barrandella, 

 were smooth, and were followed by costate genera. 



Pan-Amer. Geol., vol. 51, 1929, pp. 37-38. 

 'Geol. Surv. Canada, Mus. Bull. 3, 1914, pp. 27-28. 



