334 THE PEARLY FBESH-WATEB MUSSELS— SIMPSON. vol. xvm. 



below the IstliDius of Teliuantepec, as well as those of Cuba, are remark- 

 able for their sulcate sculpture. This character is noticeable even in 

 species which are pustulous or otherwise sculptured, and is seen in 

 groups, the members of which in Mexico are smooth or nearly so. 



Second. — A considerable number of Unios and Anodontas, some of 

 which extend down into Central America, which are either absolutely 

 identical with well-known forms in the Mississippi Valley or belong with 

 the assemblages of that region. The following groups of the latter 

 province are represented. The group of Unio pUcatiis is represented 

 by Unio eif/litsi, Lea, which is found south to Vera Cruz, and is merelj^a 

 synonym of U. nndtiplicatiis, luea, a common form in the central United 

 States, There are one or two other species of this group which range 

 south into Central America. Quite a number of species of the group of 

 Unio aJatus, such as ZTnio teco7natensis, Lea, U. umbrosium, Lea, U. pur- 

 pnratns^ Lamarck, and the like, are found in Mexico, and one species 

 something like Unio tenuissinius ( U. (lelphinulus, Morelet), is found in 

 Honduras. The group of Unio gibbosus is represented by Unio discus, 

 Lea, a compressed, ponderous species in Central America ; that of Unio 

 luteolus by a nearly typical si)ecies ?'. hydianus, Lea, and that of Unio 

 anodontoides by the form of the same name, all of which species extend 

 across the Eio Grande Eiver. Unio cou<'hianus,Tjea., of the Lachrymosns 

 group, is a Mexican species, and it is probable that representatives of 

 other northern groups will be found in this region, Anodonta henryana, 

 Lea, of Mexico, is scarcely distinct from A. imbecillis. Say, of the Mis- 

 sissippi Valley ; and the group of Anodontas, of which A. grandis, Say, 

 may be considered the type, has several representatives in the northern 

 part of the province. 



Third. — The few Glabaris and the Mycetopoda heretofore mentioned, 

 which are found in the southern part of this area. Only about a 

 half dozen species of this region are found in the United States, and 

 perhai^s as many belonging to that country extend into Mexico, 

 though these numbers will probably be increased with more thorough 

 exploration. 



DISTRIBUTION IN TIME AND GENERAL (CONCLUSIONS, 



Unio and Anodonta have been believed l)y some authors to extend 

 well back into the Paleozoic, and, while this may quite probably be 

 true, yet I do not think the evidence is sufficient to demonstrate it. 



Two or three species of Unios were collected by Professor Cope in the 

 valley of Gallinas Creek, Xew Mexico, from strata which he regarded 

 as of Triassic age.^ 



These shells were so broken as to be hardly recognizable, though 

 they are no doubt Unios. One of them, however, was described by 

 Meek under the name of Unio cristonensis,^ but it may be as well to state 



' Ann. Kept. Expl. and Sur. west of the one-linndredth meridian, 1875, p. 81. 

 2 Ann. Rept. Expl. and Sur. west of the one- hundredth meridian, 1875, p. 83. 



