304 THE PEARLY FBESH-WATEB MUSSELS— SIMPSON. vol. xviii. 



UNIO MONODONTUS, Say. 



Central part of the Mississippi Valley. The teeth are very variable. 

 Cardinals usually quite imperfect, or even rudimentary, though some- 

 times well developed. Laterals present or absent, and showing every 

 possible degree of developuient. On account of this great variation 

 the species has been placed about as often in TJnio as MargarUana. 



UNIO DECUMBENS, Lea. 



Northern Alabama and possibly adjoining States. Shell somewhat 

 resembling U. monodontiis, but shorter, wider, and with rather better 

 developed teeth. 



UNIO HEMBELI, Conrad. 



Louisiana. Very closely resembles Unio margaritiferus, but is occa- 

 sionally slightly plicate on the posterior slope. The hinge is very much 

 like that of the latter species, but in all the specimens I have seen the 

 somewhat feeble laterals are always present. 



UNIO LAOSENSIS, Lea. 



Southeastern Asia. A somewhat smaller species than U. margarit- 

 ifenis, but closely resembling it. The teeth are generally quite well 

 developed. 



UNIO CRASSUS, Retzius. 



Southern Europe. A large, very heavy species, often becoming 

 arcuate when old, with very strong, well-developed cardinals and 

 laterals. 



Conchologically and anatomically, so far as is known, the above 

 species form a very natural group. All the shells are elongated, 

 rounded before and behind; arcuate when old, without angles or 

 sculpture, except iu the case of U. hemheli; with uniform, rayless, thick, 

 dark epidermis; a curved hinge line, and a hinge iilate which is nar- 

 rowed and rounded just back of the cardinals. 



The fact of the presence or absence of lateral or cardinal teetli in 

 certain of the Xaiades can not be taken as a proof of generic distinc- 

 tion. In Java, the Philippines, and perhaps certain other islands of 

 the East Indian Archipelago, there is found a group of jSTaiades haviug 

 moderate sized, thin shells, of a peculiar lurid jiurplish or reddish tex- 

 ture, in which the prismatic layer forms a rather wide border.' 



These species, all of which have greatly compressed teeth, exhibit the 

 most remarkable variation in the degree of their development. Some 

 (»f them have perfect cardinals and laterals, others to the naked eye 

 are destitute of either, but with a glass show traces of one or both, and 



'The group is typified by Unio hengalenais, Lea, but it is doubtful whether auy of 

 the species are found on the continent. According to Hanley and Theobald (Concli. 

 ludica, p. 62), U. hengalennis does not come from India, but from the Philippines. 



