OBOLIDiE. 449 



in all cases through transitions. I was for a long time undecided as to whether I should refer all of these aberrant 

 shells to one species. Nevertheless, the fact that deviations in a much greater measure are not extraordinary in other 

 Obolus species, and, furthermore, the fear of multiplying the number of species without decisive reason, and, lastly, 

 the correspondence of the different forms in the mentioned principal characters persuaded me to look upon the entire 

 group as belonging to one species. The characteristic form was erected into a species and the others were subordinated 

 to it as varieties. However, it is possible that with more complete material the one or the other variety will be removed 

 from the species. At present this arrangement will make easy the general view of the forms. 



Diagnosis: Shells moderately large, flatly arched, drawn out lengthwise. Outline subtriangular. Tip of beak 

 of the large shell somewhat protruded, obtuse-angled, externally obliquely truncated; small shell almost rectilinearly 

 cut off. Borders of the beak of the large shell receding from the plane of shell borders, going over into the lateral 

 borders with a roundish angle. Exterior surface of shell very smooth, strongly lustrous like varnish, concentric striae 

 very fine. Growth lamellse marked by somewhat coarser grooves, commonly standing out at the anterior border like 

 shingles. Area large, splanchnocoelic part broad, band-shaped; pleuroccelic part prolonged far into the lateral bor- 

 ders. Plane of the area of the large shells broken in the peduncular groove, in the small shell depressed into broad 

 grooves between the traces of the pseudo-area. Traces of the pseudo-area parallel in both shells very near to borders of 

 area. Peduncular groove broad, flat, parallel bordered. Thickening of shell slight, inner configuration slightly pro- 

 nounced. Central depression of large shell pestle-shaped, at times indefinitely flattened. Septa and corneous proc- 

 esses receding very much. Median swelling of small shell strongly developed, divided by a sharp furrow between 

 the places of attachment of the anterior lateral muscles. Places of attachment of the muscles distinctly pronounced. 

 Those of the outside lateral muscles of large shell circular, standing off from those of the central muscles and mostly 

 separated by a roundish swelling. 



Observations: The few shells of this relatively rare species which are at hand in finest preservation do not allow 

 some necessary characters of the visceral surface to be distinguished. Especially the impressions of the lateral vessels 

 are withdrawn from observation, although just these characters, through their wonderful development and preserva- 

 tionin some species of the subgenus Schmidtia (e. g., in 0. celatus and O.'crassus), make possible in the highest degree 

 the study of the vascular system in Eichwald's genus. Yet, in spite of the incomplete characterization, 0. obtusus 

 belongs to the best-defined species of the subgenus Schmidtia. 



The peculiarity shows itself for the first in the predominant prolongation and the slight arching, which find their 

 numerical expression in the respective value of b : 1 and h : 1. These relative numbers are in 0. obtusus smaller than in 

 all other species of the subgenus. * * * 



To the longitudinally extended form now comes the broad obtuse-angled tip of the beak, which gives the shells a 

 subrectangular outline, and by which 0. obtusus is distinguished as well from all other species of the subgenus Schmidtia 

 as from those of the other subgenera. The tip of the beak of a large shell loses its blunt form, and the rounded angle 

 of the posterior border of the shell disappears, so that the outline becomes more broadly oval. In similar manner 

 the small shell is changed. The corners of the blunted tip of the beak become round, and the rectilinearly truncate 

 tip of the beak assumes a slightly convex form. 



Of less specific account than the form, but not less striking to the eye, is the more considerable size of the shells, 

 which surpasses that of all other species of the subgenus Schmidtia, the average length of the large shell being 6.47 

 mm., while the corresponding valve of the next largest species (0. crass^us) only attains 5.96 mm. These results refer 

 to the species with the exception of the varieties; of the latter, 0. obtusus longus has a still more considerable length 

 (7.7 mm.). 



The exterior smface of the shell of a specimen figured shows some short, radially placed, slight furrows on the 

 central part of the surface; the furrows are lacking on the posterior borders. This striation can, however, only be looked 

 upon as an occasional formation, for, even if slight furrows of the same kind can be noticed in another specimen, they 

 are entirely missing in all other shells examined. Isolated striae of this kind also occm' in the other Schmidtias. 



The formation of the area is very aberrant. While this part of the shell is generally triangular in the other species, 

 it assumes in 0. obtusus the form of a broad band, which hems the tip of the beak and is prolonged far into the borders 

 of the shell. The splanchnocoelic part of the base of the area runs parallel to the borders of the beak and is, corre- 

 sponding to the tip of the beak, angled in the peduncular furrow. In the small one it is rectilinear or slightly concave. 



The surface of the area of the large shell sinks, as in 0. celatus, only in a far stronger degree, from both sides toward 

 the peduncular groove; besides this the whole surface of the area inclines obliquely toward the exterior, so that the 

 borders of the beak recede from the surface of the border of the shell. The gap of the surface of the area in the pedun- 

 cular groove is partly a consequence of the deficient thickening of the posterior part of the shell, which causes the lamellae 

 of the area to appear as a mere covering of the hollow tip of the beak; yet it is partly brought about by a slight lateral 

 compression of the tip of the beak, which is also indicated at the exterior surface of the shell through a slight roundish 

 median edge perceptible in reflected light. In the small shell these conditions occur in a similar manner, but stand 

 out less distinctly. 



Especially characteristic for this species is the position of the traces of the pseudo-area, which run parallel to the 

 median line of the shell and are very near the lateral borders of the area. In some the posterior part of these ti'aces 

 seems to bend toward the tip of the beak with a roundish angle. Yet in nature the traces of the pseudo-area run recti- 

 linearly to the posterior border of the beak and the deflecting apparent prolongations of the same are only accidental 

 examples of more strongly sculptured striae of the area. 



62667°— VOL 51, pt 1—12 29 



