Eurycormus), to biconcave centra, and finally to the socket-and-ball condition that prevails at the present day.
C. Labyrinthodonta, with simple biconcave vertebral disks, very slightly pierced by a remnant of the notochord and supporting the loosely articulated neural arch.
The first compound miscroscope (discovered probably by the Middelburg lens-grinders, Johann and Zacharias Janssen about 1590) was a combination of a strong biconvex with a still stronger biconcave lens; it had thus, as well as the first telescope, a negative eyepiece.
On the other hand the Odontotormae, as exemplified in Ichthyornis, having the primitive biconcave vertebrae, yet possessed the highly specialized feature of teeth in distinct sockets.
P. 344) a second fossil bird from the same locality was indicated as Ichthyornis dispar- from the fish-like, biconcave form of its vertebrae.