BRACHIOPODA. 297 



horizontal ist, niniiiit un Hohe allmiihlich ab uiid gelit iii eine Linie iiber, wie 

 bei Productus; 7A\ gleicher Zeit venimlert sie ihre Kichtung und indeiii sich der 

 anfaiigs iiach hinten hervorragende Hacken der Oberschale allmiihlich nach 

 unten umschlagt, endlich ganz uiiter die Unterschale zu liegen kommt, hat die 

 Riickeiilinie eiiie im VerhaUnisse zu den iibrigen Terebratulen verkehrte 

 Stellung angenonunen, so dass die Beri'dirung der beiden Schalen nacli oben, 

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

 ist ghitt, mit tiachen, diinnen, durch breite ZwischenrJiume von einander ge- 

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

 Schale mehr oder weniger, oft selir 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 gdauben, man habe nur eine einzige Schale vor sich."* 



In this diagnosis given by Pander the external characters of Lept^na, 

 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,f recognized these specific terms, coml)ining many of 

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

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

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

 species of Orthis.j 



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

 forms belonging to this gi'oup must follow directly upon Orthambonites, Init we must at the same time take 

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

 lower valve, they approach more closely to Pi-oductus. The cai'dinal area, though in general triangular and 

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

 time it changes its direction and the projecting- beak of the upper valve, which at first is inclined backward, 

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

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

 is directed upwai-d an<l the beak downward [see figure 40, page 234]. The surface of the Plectambonites 

 is smooth with low, fine radiating lines separated by broad interspaces. The umbo is not very pi-ominent. 

 Upper valve more or less, often sti-ongly convex, the lower vei-y concave, and following so closely the cui-va- 

 ture of the upper that between them is a space so extremely narrow as to e:usily lead to the deception that 

 but a single valve is present." 



t Gfeologie de la Russie et des Montagues de I'Oural, pp. :3'28-232. 



X " Cette coquille, que M. Eichwald a cousideree comme nonvelle, nruis parait tellement identique avcc 

 le Plectainbonites planisaima de M. Pander, que nous n'aui-ious pas hesite a lui restituer ce nom, s'il n'avait 



