The occipital cross-commissure occurs in a well-developed form in Eryops. It is short and ends abruptly within the limits of the tabulare. Its ends are occupied by large pits. The commissure, as in Amia, grooves the postparietal and the tabulare elements. There is no evidence of an anterior commissure. I think there is evidence of a temporal canal on the left side of the skull, but am not sure. The jugal and infraorbital canals are well developed and strongly connected. The jugal canal starts far back on the supratemporal, and after curving around over the quadratojugal joins the infraorbital, or rather becomes a part of that canal, somewhere on the jugal. There is nothing unusual in the infraorbital. The antorbital commissure is well developed. It is longer and better developed than in any other known form. The supraorbital canal starts in the region of the orbit, and after curving downwards to meet the antorbital commissure, ends abruptly anterior to the nostril. There are faint traces of a lateral-line canal, the operculo-mandibular, on a poorly preserved mandible of Eryops. It does not differ greatly from that described below for Anaschisma.
Although Archegosaurus has been known for more than a century, we have had no adequate discussion of the manner of occurrence of the lateral-line canals. Burmeister ([80]) gave a figure of the canals as he thought they occurred on the cranium, but H. von Meyer ([428]) states that the representation is inaccurate, and they seem to be based largely on Trematosaurus.
The lateral-line canals occur in well-developed form on the skulls of the Stereospondylia. The sutures between the elements of the skull are usually clearly marked by smooth, narrow grooves. The lateral-line canals can always be distinguished from the sutural grooves by the shape of the bottom, being U-shaped in the former and V-shaped in the latter. The lateral-line canals also at times have their bottoms roughened by pits occurring in them; the sutural grooves always have smooth bottoms. The lateral-line canals are usually rather shallow and sometimes broad, with the edges of the grooves more or less perpendicular, but in Metoposaurus the canals are deep and the borders are sharply incised.
The temporal canals in Anaschisma from the Triassic ([49]) of Wyoming are represented by broken furrows. The portions preserved exhibit the usual downward tendency to unite with the infraorbital on the postorbital element. In its course forward from the epiotic the temporal canal cuts the squamosal. The supraorbital canal has an unusually deviating course in Anaschisma, but aside from the minor twists and curves it does not differ essentially from the same canal in other forms. It ends abruptly on the anterior end of the muzzle. In its course it gives off the vestige of an antorbital commissure which tends to join a vestige from the infraorbital canal. The jugal canal begins broadly at the very posterior edge of the skull as though it were continued, as it undoubtedly was, to the body of the animal. In its course forward it joins the infraorbital canal on the jugal. The course of the infraorbital is not unusual in any respect. There is no anterior commissure on the skull, nor is the occipital cross-commissure developed on either skull of the genus at hand.
There are distinct evidences of an operculo-mandibular lateral-line canal on the mandibles. The canal enters the mandible on the surangular and passes forward around the mandible as described for Diplocaulus ([477]).
Other members of the Stereospondylia, such as Mastodonsaurus, Metoposaurus, and Trematosaurus possess well-developed lateral-line canals, but the above description fits, in a general way, the condition in all genera; and for our present purposes that will suffice.
[CHAPTER V.]
THE AMPHIBIA OF THE DEVONIAN AND MISSISSIPPIAN OF NORTH AMERICA.