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 3'6lb. 



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. umb. dist. 



Youngest sp. transverse, '85, '73, 1*14, "7, "06. 

 Young sp. produced, 1", '86, 1'18, '77, '1. 



Largest sp. 5', 3'2, 5'8, 4'7, 1*3. 



Ovate sp. 3'65, 2*7, 4'9, 3', '55. 



Inflated sp. 3'4, 2'1, 4'2, 3'8, 17. 



llalj. Heal Llejos, Bay of Guayaquil, &c. Cuming, Hinds. 

 Ecuador, Guayaquil, Fontaine, D'Orbigny. Panama ; rare 

 but large, half buried in mud and small algse, under trees, a 

 little above half tide level ; C. B. Adams. Mazatlan ; very 

 common ; L'pool fy 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 distinct. 

 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 



