162 



GENERA OF THE SUBORDERS ORTHOIDEA AND PENTAMEROIDEA 



TECHNIQUE 



During the progress of this work, the junior author 

 has improved the old Zugmayer process of serial sec- 

 tioning so as to leave a remainder of the specimen, 

 and this is highly advantageous. Serial sectioning 

 has heretofore resulted in total destruction of the speci- 

 men, or in the destruction of so much of it that the 

 remainder was worthless. In our work we have fol- 

 lowed the course described below: Before sectioning, 

 the specimen is measured by micrometer. Sectioning 

 proceeds slowly, the structures being sketched at inter- 

 vals with the aid of a camera lucida, thus showing the 

 significant changes in spondylia, septa, and cardinalia. 

 This procedure is continued until that plane is reached 

 where the ventral teeth can be seen inserted in the 

 sockets of the dorsal valve. The specimen is then 

 cemented by balsam to a glass slide and the whole 

 measured. Sectioning is now resumed from the an- 

 terior end of the shell and continued until the internal 

 lamellae begin to appear. The specimen is measured, 

 the shell structures sketched, and this procedure fol- 

 lowed until the section is too thin to grind further, 

 when it is covered by a glass slip. In this way at least 

 a thin section of the specimen remains to form a record. 

 In sectioning from the anterior end, the distance from 

 the plane where internal structures first appear must 

 be added to the length from the beak to the plane of 

 articulation; addition of the thickness of the thin 

 slice representing the distance from the last plane of 

 the anterior sections to the plane of the articulation 

 must also be made. 



In work demanding the sectioning of brachiopod 

 material it is desirable that a replica of the specimen 

 first be made, then the original should be photographed 

 for dorsal, ventral, lateral, anterior, and posterior 

 views, and these photographs should be reproduced 

 along with those of the other type material. 



MORPHOLOGY 

 OF THE PENTAMERACEA 



The pentamerid is a rather distinctive shell, easily 

 recognized by its usually strongly biconvex valves and 

 its spondylium duplex and elongate median septa in 

 both valves. It was formerly the practice of most 

 writers to place nearly all of the spondylium-bearing 

 shells among the Pentameracea, but recent advances in 

 brachiopod studies, and especially those of Kozlowski, 

 have shown that the spondylium is a rather common 

 structure developed independently and in different 

 ways in many stocks. For example, such an unmis- 

 takable orthid as Enteletes has been shown by 

 Likharev" to have developed a spondyhum in its altera- 

 tion to Enteletella. Among the punctate spiriferids, 



- BuU. Com. Geol., vol. 45, no. 6, 1924, pp. 720-721. 



Cyrt'ina has a spondylium, and among the terebratulids 

 Amfhigenia may be cited. No one would now consent 

 to the union of these genera to the Pentameracea. 



Although the spondylium is of itself not a distinctive 

 feature of this group of shells, it is, in combination with 

 certain dorsal structures, a rather important additional 

 character. Considering the Pentameridas as a whole, 

 there is a distinctive tripartite division of the dorsal 

 septa which is characteristic of the family and appears 

 to be its most important taxonomic feature. The 

 family Camerellidas does not accord with the charac- 

 teristic pentameroid dorsal structure and it is for this 

 reason that we have given it family rank. It has 

 singular resemblances to the Syntrophiidas and when 

 material is at hand showing in detail the pallial mark- 

 ings of the Camerellidas, it may be necessary to make 

 still other family arrangements than those offered in 

 this book. Such material, in the form of internal 

 molds, is singularly lacking in both the collections 

 studied. 



We again find among the Pentameracea, as we did 

 among the Orthacea and Dalmanellacea, that dorsal 

 structures are most diagnostic of family and mutual 

 relationships. 



The following discussion is designed to summarize 

 the significant points of pentamerid anatomy. 



Exterior. — There are several features of the ex- 

 terior of the pentamerid shell that are of some interest: 



1. V entricos'tty . — Among the Camerellidas none of 

 the shells are flattish but all are rather globular and 

 the valves strongly gibbous. The Pentamerida;, on 

 the other hand, have a number of stocks that are 

 rather flattish but may have developed gibbous stocks 

 as offshoots. Such, for example, is the development 

 of the subcylindrical Pentamerus from the rather flat- 

 tish P. oblongus. C onchidium is evidently a strongly 

 gibbous stock from the start and it is difficult to ascer- 

 tain its ancestry; the best suggestion appears to be 

 that it came out of Clorinda or Barrandella. In the 

 Gypidulinas nearly all the genera have at least one 

 valve strongly arched. 



2. Ornamentation. — It is at present difficult to 

 say whether the first of the pentamerids was smooth, 

 or ribbed as is known to be the case among the orthids. 

 In the Camerellidas the species of the primitive genus 

 Camerella are partially smooth and partially ribbed. 

 It would appear safe to say that this stock started from 

 a smooth one like the syntrophiids and subsequently 

 became anteriorly ribbed, since the culminating genus 

 of the family, Anastrofhia, is the most strongly costate 

 member. 



In the Pentamerinje, on the other hand, the problem 

 is not so easy, since the ribbed and the smooth stocks 

 appear nearly simultaneously, or, as seen from present 

 knowledge, the ribbed stocks have a httle the better 

 of it from the point of view of time. Among the 

 Gypidulinas — Barrandella, Clorinda, Gyfidula, and 



