MAZATLAN BIVALVES 133 
smooth border all round. The teeth are normally numerous, 
strong, and nearly straight, scarcely shewing the middle point. 
The extreme teeth are sometimes broken into tubercles. Often 
however they become very small or even obsolete. Sometimes 
long callous processes are formed inside the line of teeth, 
which, when much developed while the teeth are obsolete, 
present a striking resemblance to the fossil genus Macrodon. 
Although the Mazatlan shells are not so large as those from 
Panama (one valve of which weighed 2°25lb, C. B. Ad.), yet a 
single pair weighed 3'6 lb. 
In the following measurements, the length is taken (1) from 
the umbo to the middle of the opposite margin, and (2) from 
the middle of the hinge to the nearest point across. The fifth 
column gives the distance between the umbos. 
long. (1) long. (2) lat. alt. winb. dist. 
Youngest sp. transverse, °85, ler 2 ay ‘06. 
“alt 
Young sp. produced, 1°, °86, PB cal: 
Largest sp.. Diz 3°2, Cuca Aizigis, leeds 
Ovate sp. 3 Oost eye AG 3° 55. 
Inflated sp. 3°4,, Dit, Ao cd: One taa 
Hais.—Real Liejos, Bay of Guayaquil, &c. Cuming, Hinds.— 
sicuador, Guayaquil, Fontaine, D’Orbigny.—Panama ; rare 
but large, half buried in mud and smail alge, under trees, a 
little above half tide level; C, B. Adams.—Mazatlan ; very 
common ; L’pool §& Havre Coll. 
Tablet 608 contains 5 specimens, square form, young.—609, 
2 sp. do. adult.—610, the largest sp.—611, 4 sp. young, slightly 
transverse.—612, 1 do. adult.—613, 1 do. large.—614, 4 sp. 
young, produced, shape approaching A. tuberculosa.—615, 1 do. 
adult.—616, 4 sp. transverse.—617, 4 sp. gibbous, young.—618, 
1 do. adolescent.—619, 2 do. adult. 
Specimens exhibiting the inside. 'Tablet 620 contains 3 sp. 
young.—621, 1 do. valves interlocking.—622, 1 do. adult, broad 
teeth, ““Macrodon” processes distinct.—623, 1 do. hinge nar- 
rower, callosity within, running from umbo.—624, 1 do. hinge 
and processes developed, margin in layers, probably from 
intrusion of dirt.—625, 1 do. teeth obsolete, processes distinet.— 
626, 1 do. hinge line narrowed off, teeth only shewing at ex- 
tremities.—627, 1 do. posterior hinge margin fractured and 
partially mended, shewing the-teeth of an earlier age. 
Feb. 1856. n 
