CONCHIFEUA. 



289 



the spire ilowuwai-ds. (Saemann). The lower valve of C. Coqumidiana is 

 sub-spiral. 



Fossil, 5 sp. U. Green-saud and L. Chalk. Bohemia, ^France, Texas. 



Fig. 212. Internal mould of Caprotina quadripartita, D'Orb. §. 

 u, left umbo; r, right umbo; I, ligamental inflection; c, cartilage; t,f, dental 

 sockets; a, a', position of adductors ; at e, a portion of the third lobe is broken away.* 

 From a specimen collected by Mr. Pratt. 



Caprotina, D'Orbigny. 



Tt/pe, C. semistriata, PL XIX. fig. 13, 14. Le Mans, Sarthe. 



Shell composed of two distinct layers ; valves alike in structure, dis- 

 similar in sculpturing ; ligamental groove slight ; cartilage internal ; riffht 

 valve fixed, striated, or ribbed, Tvith one narrow tooth between two deep 

 pits, cartilage pits several on each side of the ligamental inflection, posterior 

 adductor supported by a plate : free valve flat or convex, with a marginal 

 umbo ; teeth 2, very prominent, supported by ridges {apophyses) of the 

 adductor muscles {a, a'), the anterior tooth connected with a third plate {n), 

 which divides the umboual cavity. 



The smaller Caprotims occur in groups, attached to oyster-shells ; their 

 muscular ridges are much less developed than in the large species (fig. 212). 

 C. costata is like a little Radiolite. 



Fossil, 4 sp. IT. Green-sand, France. (The rest are Chamas, &c.) 



FAMILY IX. TridaCx\id.e. 



Shell regular, equivalve, truncated in front; ligament external; valves 

 strongly ribbed, mai'gins toothed ; muscular impressions blended, sub-central, 

 obscure. 



Animal attached by a byssus, or free; mantle-lobe extensively united; 



* The first and fourth lobes, those on each side of the ligamental inflection, appear 

 to be the two divisions of a great internal cartilage, like that of the Radiolite. (Fig. 

 205,206, c,c.) 



