| Measurements of Œstocephalus rectidens Cope. | |
| mm. | |
| Length of dentary | 22 |
| Length of tooth line | 15.2 |
| Depth of dentary at last tooth | 2.7 |
| Length of 3 centra of the caudal series | 8.6 |
| Extent of neural and hæmal spines | 8.7 |
Cope, Geol. Surv. Ohio, II, pt. II, p. 365, pl. xxxi, fig. 2, 1875.
Type: Thyrsidium fasciculare Cope.
Established on a species which presents its principal peculiarities in the structure of the vertebræ. Two specimens present inferior views of the spinal column, showing that the genus possesses, like Siren, enlarged diapophyses, but they are peculiar in their fan-like form. They resemble slightly the neural spines of the caudals of Ptyonius, but are present on the dorsal vertebræ. Whether the caudals of the present species possess ornamented neural spines the specimens do not indicate. The abdomen is protected by the usual hair-like rods arranged en chevron, the angle directed forwards. No indications of limbs can be discovered on the blocks.
Without the cranial bones the affinities of this genus can not be determined; while it may be allied to Cocytinus, the vertebræ of that form are without peculiar diapophyses.
Thyrsidium fasciculare Cope.
Cope, Geol. Surv. Ohio, II, pt. II, p. 365, pl. xxxi, fig. 2, 1875.
Type: Specimen No. 8552 G, American Museum of Natural History.