32 CASE. (Vor. XIV. 
Following are some measurements of the nuchal plate. 
Length from tip to tip of wings .. . $9 ->. . 599 meters 
Length from middle of anterior edge to ea y gceurtoy process .168 ‘« 
Thickness incenter. . . Moe, ee a eet 0 
Breadth of left wing in Beets oe. ni SSG YON, We desc ghaed bien aiekey Canam 
The peripherals are represented by eight from the left side, 
which are determinable as belonging in series, and several 
detached bones whose position is doubtful. The series extends 
from the second (?) to the ninth. The anterior, which probably 
joined the nuchal, is unfortunately lost. The second (?) periph- 
eral is slender, concave on its outer edge, and bears no facet 
for arib. It articulates strongly with the third. The third is 
strong and broad, and bears a deep pit for a rib near its 
external margin. The thickened external margin is turned 
slightly downward and inwards. 
In the succeeding peripherals the length becomes greater 
than the breadth, and the external margin becomesacute. The 
turning in of the margin begun on the third becomes broader 
and broader, shoving the pit before it till in the posterior 
peripherals it occupies half the under surface of the bones, and 
its anterior edge underlies the centrally located pit. The inner 
edges of the peripherals are irregular, and extended into slender 
processes. Pl. V, Fig. 11, shows the upper surface of the fifth 
and sixth, and gives a good idea of the general shape of the 
peripherals and their strong articulation one with another. 
The following measurements are accurate in so far as the 
broken condition of the inner edge would permit. 
Length second (?) pone See ae reel O pILELETS 
Breadth ‘“ moran (84 fou sam Sesion » ie 
Length third x el et Soe deere zO2 Re 
Breadth ‘“ ue ROM nh rok avon 
Length fourth se MMC ar S ditetoe fe 
Breadth “ ey Siete s oo EA 
The condition of the carapace of Protostega as described 
above is heralded in the young of Chelone Benstedii Owen, 
where the costal plates taper from the proximal to the distal 
end, and in Al/opleuron Baur (Chelone Hoffmani Gray), where 
the ribs have become very slender and the costalia short and 
