INVERTEBRATE PALAEONTOLOGY. L>l>:5 



hiatus, formed by ;i similar but deeper rectangular notch, which extends from 

 the base nearly balf-way up to the beaks, and back almost to the middle of 

 the valves; base between the anterior and posterior notches, extended down- 

 ward in the form of a narrow prolongation, which curves under, and is the 

 only part of the ventral borders of the two valves that comes in contact ; beaks 

 elevated, gibbous, incurved, and located between the middle and the anterior 

 margin. Surface ornamented by small concentric lines, which are curved 

 and deflected parallel to the great irregularities of the free borders, and 

 crossed by two radiating grooves, the posterior of which passes from the back 

 part of the beaks obliquely downward and backward to the corner of the 

 posterior notch ; and the other, which is more distinct, extends almost 

 directly downward to the extremity of the ventral prolongation. 



Length, of a medium-sized specimen, 0.16 inch ; height, 0.14 inch ; 

 convexity, 0.13 inch. 



On the posterior side of the shell, the concentric lines, or wrinkles, 

 are comparatively large, rather irregular and depressed ; but where deflected 

 upward parallel to the straight vertical margin of the anterior hiatus, they 

 become very fine, regular, closely arranged, and minutely and beautifully 

 crenulate. On being suddenly deflected horizontally forward upon that part 

 of the anterior extending over the hiatus, they become again suddenly 

 enlarged, but continue to be very regular, and seem to retain some traces of 

 crenulations. 



The posterior oblique groove is so much larger and deeper on internal 

 casts than on the exterior of shell, as to indicate the presence of a corre- 

 sponding strong ridge on the inside of the valves ; while the other nearly 

 vertical groove is smaller and less distinct than on the external surface. 



The tubes are rather small, nearly straight, or sometimes a little bent; 

 and increase gradually from one extremity to the other. They usually pene- 

 trate the fossil wood in which they are found, nearly parallel to each other, 

 and are often crowded so as to leave narrow spaces between. They vary in 

 size from 0.10 to 0.20 inch in diameter, and sometimes seem to have been 

 provided with a few incomplete, irregular, transverse septa, near the smaller 

 or outer extremity. 



Locality and position. — Fort Clark, on the Missouri, Dakota Territory ; 

 in the Fort Pierre group, or formation No. 5 of the Upper Missouri Creta- 

 ceous series. 



