568 MEMOIRS OF THE CARNEGIE MUSEUM. 



in common with the Hyriina: that the Mukiinw arc rchited to, and prol^ably des- 

 cended from, the latter; but connecting hnks which undoubtedly stand between 

 these two subfamilies are as yet unknown, and only the presence among the Muteli- 

 ncc of forms with hinge-teeth suggests that there once was a closer connection with 

 the Hyriince. According to our present knowledge, the two subfamilies are un- 

 doubtedly allied; but they are very sharply separated by anatomical as well as 

 shell-characters, and it is impossible to form an appropriate idea of their genetic 

 connection. 



Simi^son (1914, p. 1384 et seq.), in his family Mulelidce, admits six South 

 American genera: Monocondijla'a, Iheringella, Foi:<sula, Leila, Anodontites, and 

 Mycetopoda. These are easily distinguished by shell-characters, which are tabu- 

 lated in the following key. 



Key to the South American genera of Mutelin.e. 

 a I. Hinge with more or less developed pseudocardinal teeth. Lunula short or almost absent, not much 

 produced in front of the beaks. 

 hi. Hinge-plate rather broad, with oblique, compressed or tul)ercular, alternating pseudocardinals, 



two in each valve, the most anterior in the left valve Iheringella. 



bi. Hinge-plate narrow, with the pseudocardinals stumpy, squarish, or depressed, left valve never 

 with two distinct pseudocardinals. 

 Ci. Left valve with one, right with two stumpy, or squarish, pseudocardinals, which alternate, 



the most anterior one being in the right valve Fossuln. 



Co. Either valve with only one pseudocardinal, which is generally depressed (spoon-shaped), 



that of the left valve being the most anterior Monocondylwa. 



Ui. Hinge without any pseudocardinal-teeth. Lunula elongated and much produced in front of the 

 beaks, forming a kind of anterior ligament. 

 bi. Pallial line without a sinus behind. 



f 1. Shell rounded, subovate, or subtrapezoidal. Valves closed or somewhat gaping (when the 

 shell is elongated, the valves are closed). Anal opening entirely open. Branchial opening 



not closed in front. Foot normal Anodontites. 



f2. Shell elongated, gaping in front. Anal opening closed above. Branchial opening said to 

 be closed in front (liiis is iloul)tful). Foot very long, ileveloped at its distal end into a 



sort of button Mijceto/mda. 



b-2. Pallial line with a sinus behind. Shell large, subovate, winged. \'alves gaping. Branchial 

 opening said to be closed in front Leila. 



With regard to the genetic connections of these genera, it is clear that we are 

 to regard the first three as the more primitive forms, and, judging from the shell, 

 Ihermgella appears to Yw the most primitive, having the most complete hinge. 

 There is no doubt that of the others Anodontites is the simjilest, but its connection 

 with the first three is obscure, since the shell-characters do not connect it more 

 closely with any one of them. Both Mycetopoda and Leila seem to have descended 



