358 KANSAS UNIVERSITY SCIENCE BULLETIN. 



fairly regular intervals. The upper one or two tubercles of 

 the posterior series are sometimes lacking. 



Quadrula houstonensis Lea. Not figured. 



Unio houstonensis Lea. Proc. Acad. Nat, Sci. Phila. , III, 1859, p. 155. 



Shell large, thick, moderately inflated, very roundly quad- 

 rate, slightly broader below than above. Anterior margin 

 straight or very slightly curved above, abruptly rounded be- 

 low ; ventral margin regularly curved ; posterior margin 

 slightly curved and set at about right angles to the ventral 

 margin ; dorsal margin straight or slightly curved, oblique. 

 Umbones large, full, high, directed a little anteriorly. An- 

 terior and lateral umboidal slopes full and rounded ; poste- 

 rior slope somewhat truncate and very slightly incurved. 

 Epidermis dark yellow, with smooth, rounded and regular 

 lines of growth, which maybe imbricated posteriorly. Liga- 

 ment high, thick, and of moderate length. Lunule large. 



Interior : Pseudocardinals of moderate size and light, 

 double in the left and single ( with sometimes a small ante- 

 rior auxiliary) in the right valve ; the left pseudocardinals 

 being low and pyramidal, and arranged as the legs of a broad 

 A, the right arising from a deep, ragged pit. Interdentum 

 broad and long. Laterals short, thick, very oblique, curved, 

 highest posteriorly. Anterior adductor cicatrix deep, not 

 large, elongate, roundly triangular in outline. Posterior 

 scars rather small, well impressed, distinct. Dorsal scars in 

 a long row on the under surface of the interdentum and 

 pseudocardinal. Pallial line well impressed its entire length. 

 Cavity of the shell moderate, of the beaks deep and com- 

 pressed. Nacre silver-white, slightly iridescent posteriorly. 



Length. Height. Breadth. Um. ra. 



73 70 48 0.13 (46.1) 



This species is found in Texas, Louisiana, southern Arkan- 

 sas, and Kansas. It is reported from Mill creek, in the Kan- 

 sas system (Call), and in the Verdigris (Call) and Spring 

 river. It is not a common species. It resembles smooth 

 specimens of Q. pustulosa, but is smooth, heavier and larger 

 than that species. 



