BRACHIOPODA. 
297 
horizontal ist, nimmt an Hohe allmahlich ab und geht in eine Linie liber, wie 
bei Productus; zu gleicher Zeit verandert sie ihre Richtung und indem sich der 
anfangs nach hinten hervorragende Hacken der Oberschale allmahlich nach 
unten umschlagt, endlich ganz unter die Unterschale zu liegen kommt, hat die 
Riickenlinie 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, dlinnen, 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^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. pianissimo,, must be taken as the type of the genus. 
De Yerneuil, 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. pianissimo, P. crassa, and P. testudinata, and remarks that 
he would have adopted the specific term pianissimo had it not been in use for a 
species of ORTHis.f 
* “ The cardinal 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 upward and 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 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 extremely narrow as to easily lead to the deception that 
but a single valve is present.” 
t G6ologie de la Russie et des Montagnes de l’Oural, pp. 228-232. 
+ “ Cette coquille, que M. Eichwald a consideree comme nouvelle, nous parait fellement identique avec 
le Plectambonites pianissimo de M. Pander, que nous n’aurions pas hesite a lui restituer ce nom, s’il n’avait 
