GENERA OF THE SUBORDER ORTHOIDEA 



131 



growth stages, as is so forcibly indicated by the occur- 

 rence in the early Cambrian of Nisusi/i and the later 

 Billingsellidx with their covered delthyria. 



Beecher'"" has suggested the derivation of BUob'ttes 

 "from a radicle haying, in many respects, the charac- 

 ters of the group represented by PLitystrophia bijorata. 

 This statement has been construed by Cumings and 

 others as "a virtual demonstration of the derivation of 

 BUob'ttes from Platystrophia." Beecher and Cumings 

 showed the external resemblances between the nepionic 

 stages of the two genera, but the former merely pointed 

 out that the two may have come from a platystrophoid 

 radicle. This is true from the standpoint of exteriors, 

 but certainly the internal features of Bilobiies and 

 Platptrofhia are diverse. On an earlier page it has 



Genus MYSTROPHORA Kayser 1871 



PI. 16, figs. 1-5; t. figs. 20, 21 



Kayser, Zeits. deut. geol. GcscU., vol. 23, 1871, pp. 612- 

 614, pi. 13, fig. 5. 



Genolectotype (Williams and Breger 1916). — 

 Orthis areola Quenstedt 1871, Petref. Deutschl., p. 

 589, pi. 57, fig. 27. 



Description. Exterior. — Transversely subquad- 

 rate to subsemicircular in outline; cardinal extremi- 

 ties obtuse or angular; lateral profile plano-convex; 

 anterior commissure sulcate, sulcus deep, ventral fold 

 low, marked by a median costella. Ventral palintrope 

 long, strongly apsacline, beak slightly curved; del- 



^ ^ ^ <y <y <x> <o 



Fig 20 —Myj/ro/>Aor<j ar^o/d Quenstedt.— Section 1 shows the peculiar apical plate, which is punctate; 2 shows the 

 relationship of this plate to the dental lamelli; 4 and 5 show the long brachial processes; and the last two sections illustrate 

 the strong median septum. Distance of sections from beak: 



-0.8 mm. 

 -1.5 

 -3.2 

 -3.8 



5 — 4.7 mm. 

 6—6.7 



7—7.1 



The complete specimen was 10 mm. long. The median septum thus divides the shell into two chambers, x about 2. 



been shown that Platystrophia is most closely allied to 

 Plectorthis and Hebertella, that its cardinalia form a 

 distinct type among impunctate orthids. The test of 

 Bilobites is punctate and its whole expression is dal- 

 manelloid. It would seem, then, that Bilobites has 

 been derived from a dalmanelloid stock that introduced 

 deep emargination of the anterior margin, a strong 

 sulcus in each valve, and the enlargement of the primi- 

 tive bladelike Dalmanella brachiophores. Accordingly 

 any origin from Platystrophia is held to be untenable. 



Family MYSTROPHORID^ Schuchert and 

 Cooper 1931 



Aberrant and specialized Dalmanellacea paralleling 

 SkenidioiAes internally. The brachiophores form a 

 cruralium and the cardinal process is lobate. The only 

 certain genus of the family is Mystrophora Kayser, 

 which appears to have had its origin in the Dalman- 

 ellida:. We provisionally refer here also the genus 

 Kayserella. Both genera are of the Middle Devonian 

 of Germany. 



""> Op. cit., p. 404. 



'"Amer. Jour. Sci. (4), vol. 15, 1903, p. 40. 



thyrium usually open but may be closed at the apex 

 by callus. Dorsal interarea short, plane, anacline; 

 notothyrium partially closed by the cardinal process. 

 Surface finely multicostellate ; shell fibrous, punctate. 



Ventral interior. — Delthyrial cavity deep; dental 

 plates low, divergent. At the apex between the dental 

 plates there is a flat callus deposit, probably for pedicle 

 attachment. The cavity beneatli this is usually com- 

 pletely filled by punctate shell substance. 



Dorsal interior. — Notothyrial cavity deep; cardinal 

 process with a long shaft and lobate myophore. Brach- 

 iophores elongate, pointed. Brachiophore supports con- 

 vergent and uniting with a median septum, which is so 

 high as to divide the shell into two chambers. Muscu- 

 lature unknown. 



Geologic range. — Middle Devonian. 



European Species 



Orthis areola QncnUcil 1871 



O. deshayesi Rigaux 1873 



Scenidium jallax GUrich 1 896 | p^^yication not seen 



S. folonkum Ourich 1 896 ) 



Distinguishing characters. — The name My- 

 strophora has usually been regarded as a synonym of 

 Skenidium, despite the fact that its author showed that 

 some of the shells to which he applied the name were 



