386 



PROCEEDINGS OF THE NATIONAL MUSEUM. 



The American species of Oholus now known to me are: 

 American species of Obolus. 



Name of species. 



Obolus aneeps Walcott 



Ohohis loperi Walcott 



Oholus mmra Hall and Whitfield 



Oholus inaiinalis Hall 



Oholus mictwitzi Walcott 



Obolus murrayi Billings 



Obolus pandemia Walcott 



Obolus namouna Walcott 



Obolus refuhjens Matthew 



Obolus rhea Walcott 



Cambrian. 



L. 



M. 



Lower 



Ordovi- 



clan. 



OBOLUS MICKWITZI, new species. 



General form rounded ovate, with the ventral valve broadly sub- 

 acuminate, and the dorsal valve obtusely rounded; valves, as shown 

 by the casts, moderately convex, which would j;ive a rather strongly 

 convex shell, as fragments show that it was quite thick over the cent- 

 ral portions. Fragments of the shell showing the outer surface indicate 

 that it was marked by concentric lines and stri.c of growth; radiating 

 stri.TB may have been present; they are strongly developed when tlie 

 outer surface is exfoliated; the casts of the interior of the valves show 

 very strongly concentric undulations and lines of growth, although 

 in some specimens these characters are scarcely perceptible. The 

 fragments of the shell preserved show that it was formed of a thin 

 outer layer, several inner layers or lamelhe of varying thickness, and 

 numerous lamelhe over the anterior and lateral portions of the shell 

 that are slightly oblique to the outer surface. A somewhat rounded 

 ventral valve has a length of 9 mm.; width, 9 mm.; a dorsal valve 9 

 mm. long has a width of 8 mm.; a more elongate ventral valve is 9 mm. 

 in length and 8 mm. in width; an associated dorsal valve 7.5 mm. in 

 length has a width of 7 mm. 



Casts of the interior of the ventral valve show an area of medium 

 length, divided midway by the cast of a strong, rather broad, pedicle 

 furrow, and again a short distance each side of the pedicle furrow by a 

 narrow, sharp, flexure line; stride of growth cross the area of the 

 pedicle furrow parallel with the front margin. There is slight evidence 

 in one of the casts that the area formed a shelf between the pedicle 

 groove and the lateral margin. The area of the dorsal valve is of 

 medium length and fairly well extended out on to the cardinal slopes. 

 The cast of the visceral cavity is well shown by several S])ecimens. It 

 resembles that of 0. matinalis and 0. quenstedi in the extension of the 

 anterior margins almost directly outward from the center toward the 

 impression of the main vascular sinuses; one of the peculiarities of the 

 species is the great development of the area within the parietal scar 

 (Splanchnocoele); in some examples it occupies all the central portions 



