In the Psammosteidæ of the Devonian the head is covered with large plates which are not penetrated by the sense-organs. These plates are covered with minute, close-set tubercles, covered with brilliant ganoid enamel and with finely crimped edges. According to Dr. Traquair, these tubercles are shagreen granules which have coalesced and become united to plates formed in a deeper layer of the skin, as in Ateleaspis the minute scales have run together into polygonal plates. These creatures have been considered as "armored sharks," and Dr. Traquair regards them as really related to the acanthodean sharks. Nevertheless they are not really sharks at all, and they find their place with the Pteraspis and other longer known Heterostracans.
The family of Drepanaspidæ consists of a single recently known species, Drepanaspis gmundenensis, found in a pyritized condition in purple roofing-slate in Gmünden, Germany. This fish, which reaches a length of about two feet, has a broad head, with eyes on its outer margin, with a slender body and heterocercal tail. The head has a broad median plate and smaller polygonal ones. The flaps, supposed to represent the pectoral fins, are here cased in immovable bone. No trace of internal skeleton is found by Traquair, who has given the restoration of this species, but the mouth has been outlined.
Fig. 358.—Drepanaspis gmundenensis Schlüter. Upper Silurian, Gmünden, Germany. (After Traquair.)
The best known of the Heterostracan families is that of Pteraspidæ. In this family the plates of the head are coalesced in a large carpace, the upper part originally formed of seven coalesced pieces. A stout dorsal spine fits into a notch of the carapace. The slender body is covered with small scales and ends in a heterocercal tail. The dermal sense-organs are well developed. Pteraspis rostrata occurs in the Lower Devonian. Other genera are Palæaspis and Cyrthaspis.
Fig. 359.—Pteraspis rostrata Agassiz. Devonian. Family Pteraspidæ. (After Nicholson.)
Order Osteostraci.—The Osteostraci (ὄστεον, bone; οστρακος, box) (called Aspidocephali by Rohon) have bone-corpuscles in the shields, and the shield of the back is in one piece without lateral-line channels or sense-organs. Ventral shield single. The order includes three families. The Cephalaspidæ have the shields tuberculate, the one between the eyes fixed, and the anterior body-shields are not fused into a continuous plate. The best known of the numerous species is Cephalaspis lyelli from the Lower Devonian of England. Hemicyclaspis murchisoni occurs in the Upper Silurian of England, and the extraordinary Cephalaspis dawsoni in the Lower Devonian of Gaspé, Canada. Eukeraspis pustulifera has the head-shield very slender and armed with prickles. In the Thyestidæ the anterior body-scales are fused into a continuous plate. Thyestis and Didymaspis are genera of this type. The Odontotodontidæ (Tremataspidæ) have the shield truncate behind, its surface finely punctate, and the piece between the eyes not fixed. Odontotodus[155] schrenki is found in the Upper Silurian of the Island of Oesel in company with species of Thyestes. The Euphaneropidæ are represented in the Devonian of Quebec.
Fig. 360.—Cephalaspis lyelli Agassiz, restored. (After Agassiz.)