128 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I48 



septum duplex, extending anteriorly nearly half true length of valve, 

 about one-third length measured along floor of valve. 



Brachial valve interior with camarophorium beginning as swelling 

 of median septum in apex, widening anteriorly to form shallow trough, 

 connected to base of hinge plate by short intercamarophorial plate ; 

 median septum extending anteriorly along floor slightly less than half 

 length of valve, slightly beyond midlength of shell ; longitudinal curva- 

 ture of camarophorium not great, keeping well away from pedicle 

 valve spondylium ; hinge plate short, slightly curved ventrally away 

 from camarophorium ; crura not observed. 



Holotype. — N.Y. State Museum No. 1172 (designated by Fager- 

 strom, 1961, p. 29). 



Growth. — The width of shells of this species increases proportion- 

 ately with increase in length, although nearly all measured shells are 

 narrower than they are long (fig. 30). Increase in thickness, on the 

 other hand, is somewhat accelerated relative to increase in length, 

 with points on the diagram falling along a rather well-defined curve. 

 Projection of either of the two trends toward a low number on the 

 length-axis indicates probable allometric growth in growth stages 

 earlier than those of specimens available for study. Very small shells 

 probably are subequal in length and width, but very flat. 



Comparisons. — Coledium rhomboidale is characterized by its elon- 



Table 27. — Measurements of Coledium rhomboidale from the Logansport Lime- 

 stone near Pipe Creek Falls, Ind. 



Measurements, in millimeters 



