6 



AGNATHA. 



and a pair of large cornua (c) on the sides. A rudiment of a median 

 dorsal spine (s) also appears behind. The orbits are not completely 

 enclosed in the shield, and notches for them are rarely seen. The 

 markings on the visceral aspect of the shield are described above (p. 

 4, fig. 2). These fossils are always small, never more than about 

 0'05 m. in length, and form perhaps the earliest satisfactory evidence of 



Cyathaspis banksi ; diagram of dorsal shield, external aspect, slightly reduced. 

 U. Silurian ; Herefordshire, c, cornua ; d, median disc ; o, position of 

 orbit ; r, rostral plate ; s, posterior dorsal spine. (After Lankester.) 



chordate animals hitherto discovered. One species is recorded from the 

 Wenlock Limestone of the Island of Gothland. The typical species, 

 Cyathaspis banksi, occurs in the Ludlow Bone-bed and the Downton 

 Sandstone of Herefordshire; while the ventral shield named Pteraspis 

 ludensis, from the Lower Ludlow of Leintwardine, also probably belongs 

 to this genus. Other species are met with in the Lower Old Red Sand- 

 stone of Herefordshire, and in the Upper Silurian of Galicia and New 

 Brunswick. Cyathaspis integer, from a supposed Upper Silurian boulder 

 found near Berlin, was the first species to exhibit the dorsal and ventral 

 shields in association. 



Palaeaspis (Holaspis). A simple dorsal shield resembling that of 

 Pteraspis in form, but only notched by the orbits, and without posterior 

 spine. One species, Palceaspis (Holaspis) sericea, from the Lower Old 

 Red Sandstone of Monmouthshire, beautifully displays the arrangement 

 of the sensory canals. Another species, P. americana, occurs in the 

 Upper Silurian (Onondaga Group) of Perry Co., Pennsylvania, and is 

 known by associated dorsal and ventral shields. 



Pteraspis (figs. 4 6). The only genus in which traces of the scaly 

 caudal region have been observed. The dorsal shield (fig. 4) is arrow- 

 head-shaped, and consists of seven separately calcified portions. There 

 is a large central disc (d), with a triangular rostral plate (r) anteriorly 



