BRITISH FOSSILS. 75 



thafi fig. 4, and having an overhanging rounded lobe in front above 

 the insertion of the teeth. Some specimens agreeing with this in 

 proportion have the basal lobe of the palpi broader. 



Swimming Feet (Ectognaths), Plate VI. figs. 1 to 3, Plate VII. 

 figs. 1 to 3. The large basal joints of these organs are among the 

 most characteristic parts of Pterygotus, and were at first supposed 

 to be the mandibles or maxillae, as they are most frequently found 

 detached from the other joints of the limb. Agassiz figured a 

 portion as a part of the tail flap, and we reproduce his original 

 specimen, Plate VII. fig. 3. It will be best to describe this portion 

 first. It is the 



Basal Joint (Coxognathite), Plate VII. figs. 1 to 3. These are six 

 inches and a half and even seven inches long ! and not less than three 

 inches broad, gently curved (flask-shaped) convex, with the outer 

 or forward edge thickened, and concave on the inner or hinder side, 

 with the large basal portion almost orbicular, but narrowing into 

 the neck, and the terminal lobe again somewhat broadened and sub- 

 rhomboidal, with the serrate border placed obliquely to the general 

 direction of the entire joint. 



The teeth are twelve in number, gradually longer from behind 

 forwards, the first often five lines long, and three broad at base. 

 They are conical, a little curved and grooved down the middle, but 

 not at all articulated to the base. Being set on a straight margin 

 and regularly increasing in size forwards, they present a serrate 

 edge along the points, rounded oft" a little behind into a very small 

 basal lobe 6. There are the same number of teeth in young as in 

 mature specimens. 



Large tubercular plicae cover the whole surface, except the inner 

 thin margin of the terminal lobe, and these are concave everywhere 

 to the border, except towards the inferior angle behind the attach- 

 ment of the foot (Plate VII. fig. 2 c), where they present their 

 convex side to the margin ; and hence this portion is easy to 

 identify, even in fragments. 



The limb beyond the large basal joint is very suddenly contracted. 

 The point of attachment for these is very conspicuous, as a deep 

 notch in the foliaceous base, about half way up on the outer side, 

 and from this point to the upper angle of the serrate tip is the line 

 of greatest convexity. The basal edge is rounded below, but angular 

 where it joins the inner margin. 



The other joints of the limb are represented in Plate VI. figs. 1, 2 ; 

 and we are able to give a more complete account of them in this 

 than in any other species ; fig. 1 represents the upper and fig. 2 the 

 lower surface of the limb. 



