BRACHIOPODA. 297 



horizontal ist, niniiut an H(')he allmiihlich ab und geht in eine Linie iiber, wie 

 bei Productus; zu gleicher Zeit veraudert sie ihre Richtung und indem sich der 

 anfangs nach hinten hervorragende Hacken der Oberschale allmiihlich nach 

 unten uinschliigt, endlich ganz unter die Unterschale zu liegen konunt, hat die 

 Riickenliuie eine im Verhaltnisse zu den iibrigen Terebratulen verkehrte 

 Stellung angenommen, so dass die Beriihrung der beiden Schalen nach oben, 

 der Hacken nach unten zu stehen kommt. Die Oberliache der Plectamboniten 

 ist glatt, mit flachen, di'innen, durch breite Zwischenraume von einander ge- 

 trennte Langsstreifen versehen ; der Hacken ragt sehr wenig hervor. Obere 

 Schale mehr oder weniger, oft sehr stark convex, die untere sehr concav, und 

 ahmt so sehr die Wolbung der oberen nach, dass zwischen beiden ein iiusserst 

 kleiner Zwischenraum iibrig bleibt, ja dass man selbst verleitet werden konnte 

 zu glauben, man habe nur eine einzige Schale vor sich."* 



In this diagnosis given by Pander the external characters of LeptjEna, 

 Davidson (not Dalman), are very forcibly described, and although the internal 

 features are not detailed, the generic figure, given on plate iii, figure 8, of the 

 work cited, shows quite distinctly the peculiar cardinal process, crural plates 

 and conspicuous muscular scars characteristic of the brachial valve of this 

 group. Pander described twelve species of Plectambonites, the first of which, 

 P. planissima, must be taken as the type of the genus. 



De Verneuil, in lS45,t recognized these specific terras, combining many of 

 them, though retaining several of the original names. Under Leptana convexa. 

 Pander, he included P. planissima, P. crassa, and P. testudinaia, and remarks that 

 he would have adopted the specific term planissima had it not been in use for a 

 species of OfiTHis.f 



*" The cardinal area [postei-ior surface] is again triangular, and judging from its outline alone, the 

 forms belonging to this gi-oup must follow directly upon Oi'thambonites, but we must at the same time take 

 into consideration the entire form of the shell, and thus we see that, mainly from the strong concavity of the 

 lower valve, they appi'oach more closely to Pi-oductus. The cardinal area, though in general ti-iangular and 

 at tirst nearly horizontal, gradually decreases in height and becomes linear, as in Productus ; at the same 

 time it changes its direction and thi- pi-ojecting- beak of the upper valve, which at tirst is inclined backward, 

 becomes gradually incurved and finally comes to lie wholly beneath the lower valve, so that the cardinal line 

 has, in relation to "the other Terebratulas, a reversed position, and hence the surface of contact of the valves 

 is directed upwai-d and the beak downward [see figure 40, page 234]. The sui-face of the Plectambonites 

 is smooth with low, tine radiating lines separated by broad interspaces. The umbo is not very prominent. 

 Upper valve more or less, often strongly convex, the lower very concave, and following so closely the curva- 

 ture of the upper that between them is a space so exti'emely nai'i-ow as to esisily lead to the deception that 

 but a single valve is present." 



t Geologic de la Russie et des Montagues de I'Oural, pp. 228-232. 



I " Cette coquille, que M. Eichwald a consideree comme nouvelle, nous paraittellement identique avec 

 le Plectambonites planisnima de M. Pander, que nous n'aui'ions pas hiisite a lui restituer ce nom, s'il n'avait 



