520 THE PALEONTOLOGY OE MINNESOTA. ,i,>„i».tum 



[Orthodesma canaliculatum. 



Orthodesma canaliculatum. 

 plate xxxvii, figs. 7-11. 



Shell elongate, the length three times the hight; cardinal and basal margins 

 stiraight, nearly parallel; posterior margin oblique, rounding into the hinge line, below 

 which it slopes backward with a gentle curve to the postero-basal extremity where 

 it turns, abruptly into the basal line; anterior end contracted in front of the beaks, of 

 moderate length, rounded, most prominent a little above the middle. In a side view 

 the beaks project very little, are compressed by a broad shallow sulcus which crosses 

 the valves and occupies a large part of the anterior three-fifths of the shell; umbonal 

 ridge rather distinct, extending from the beaks to the postero-basal extremity. In 

 a cardinal view of casts of the interior, the only condition in which the species has 

 been noticed, the hinge line is strongly depressed, lying at the bottom of a wide and 

 deep channel, deepest between the rather widely separated beaks and gradually, 

 shallowing posteriorly. Casts usually almost smooth, exhibiting only a small num- 

 ber of obscure concentric furrows. One specimen preserves a small part of the shell 

 and this shows that near the dorsal edge the outer surface is marked with somewhat 

 regular raised lines, about six of them in 5 mm. The best preserved casts exhibit in 

 the posterior half of the mesial sulcus a number of obscure radii. Anterior muscular 

 scar sharply defined at the inner side, rather small, broad-oval or circular, occupying 

 the middle two-fourths of the upper half of the anterior end. Posterior impression 

 somewhat larger than the anterior, subcircular, with a narrow prolongation extend- 

 ing forward nearly parallel with the posterior cardinal margin. Pallial line distinct 

 in the anterior half, consisting (on the casts) of a straight row of obscure pustules 

 extending in a slightly oblique direction from the base of the anterior adductor 

 impression toward a point much laearer the ventral border. 



There are several peculiar features about this species. (1) I have never seen its 

 valves separate, a fact indicating, if it is not fully accounted for by the next circum- 

 stance, a strong ligament. (2) Its natural position seems tD have been with the 

 anterior end down, and so it is commonly found in the shales, and in consequence 

 it is often greatly shortened by pressure. (3) The channel-like depression of the 

 hinge; and (4) the unusual course of the anterior half of the pallial line. These 

 peculiarities distinguish the species readily from all others of the genus known. 



Formation and locality.— UuAson Eiver group, Spring Valley, Minnesota. Fragments have been 

 found at many localities in Oliio and Indiana where the upper beds of the Cincinnati formation are 

 exposed, Good specimens, however, are very rare. 



