BRACHIOPODA. 297 



horizontal ist, nimmt an H(>he allmahlich ab und geht in eine Linie iiber, wie 

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

 anfangs nach hinten hervoi'ragende Hacken der Oberschale allmilhlich nach 

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

 Rlickenlinie 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 Oberflache der Plectamboniten 

 ist glatt, mit flachen, diinnen, 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 ausserst 

 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 LEPT.ffiNA, 

 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 1845,f recognized these specific terms, combining many of 

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

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

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

 species of ORTHis.f 



*" The cai'dinal area [posterior surface] is again triangular, and judging' from its outline alone, the 

 forms belonging to this group must follow directly upon Orthambonites, 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 approach more closely to Productus. The cardinal 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 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, fine radiating lines separated by broad interspaces. The umbo is not very prominent. 

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

 tui-e of the upper that lietween them is a space so extremely narrow as to easily lead to the deception that 

 but a single valve is present." 



t G6ologie de la Russie et des Montagues de I'Gural, pp. 228-232. 



t " Cette coquille, que M. Eichwald k consideree comme nonvelle, nous par.ait tellement identique avec 

 le Plectambonites 'planissima de M. Pander, que nous n'aurions pas hesite a lui restituer ce nom, s'il n'avait 



