OBOLID^. - 443 



of the large shell somewhat receding from the plane of the lateral borders. Beak and anterior border of the small shell 

 somewhat projecting beyond the same. Shell borders thin, sharp edged, in the small shell formed like the border of 

 a plate. Outer surface of the shell polished, somewhat in'egular because of unequally deeply cut concentric furrows, 

 and rough toward the front because of the projecting borders of the lamellae. Area small, in the large shell triangular; 

 in the small, shaped like a band. Splanchnoccelic part of the large shell reduced to the borders of the peduncular 

 grooves; with the small excavated into narrow furrows. Pleurocoelic parts of the large shell deeply hollowed out; in 

 both, at the base, posteriorly somewhat caudate, extended, and drawn out into the lateral borders. Peduncular groove 

 parallel-bordered, narrowing as a rule toward the apex of the shell in the excavation of the area supported by a bridge. 

 Traces of a pseudo-area on the large shell coinciding with the borders of the peduncular groove or running very close 

 to the same; in the small shell in similar position, including between them the narrowly grooved, excavated, splanch- 

 noccelic part of the area surface. Shell thickening slight. Median septum under the magnifying glass mostly 

 perceptible, in the large shell often in connection with the bridge supporting the peduncular fuiTOW. Lateral septa 

 receding. Median swelling of the small shell plainly developed, between the places of attachment of the anterior 

 lateral muscles in the form of a sharp ridge. Posterior part of the central depression very widely extended; anterior 

 part parallel-bordered, at the tip scarcely broadened. 



Observations: 0. acuminatus is, next to 0. obtusus, the most characteristic species of the subgenus. The most 

 essential differences of these two species, which lie in the formation of the beak, have been referred to in the beginning. 

 For the rest they also differ so far from each other in the form and size of their shells that a confusion of them is pre- 

 vented. The external charactera which separate 0. acuminatus from the two other species of the subgenus Schmidtia 

 are less striking. 



From 0. celatus the species now in question is separated externally principally through the oval outline, in the 

 more considerable height of their shells, whose gi-eatest height lies more toward the beak; fi-om 0. a-assus, to which it 

 is closely related in relation to the outline and height of the shell, through the steeper falling off of the arching toward 

 the tip of the beak. The shells of the last-named species have likewise an oval outline, but the relative breadth of 

 the sam* is greater. Also the arching of the shells in 0. crassus is somewhat stronger in the middle, but the greatest 

 height lies more toward the center of the shell. The S-shaped section of the border of the large shell of 0. acuminatus, 

 to which a slight concavity in the small shell corresponds, was also noticed in 0. celatus pr^cisus; but here also the 

 broad oval outline of the shells of this variety and the flat falling off of the arching of the tip of the beak give a certain 

 and distinctive external character. 



The relations are shown more clearly by comparing the average characters of the respective relative numbers 

 than by figures and descriptions. 



The aberrant characters, which are present in the interior of the tip of the beak, ai-e by far more important for 

 the distinction of the species than the external differences of form. The most characteristic among these is the position 

 of the traces of the pseudo-area, which in 0. acuminatus run parallel and coincide with the peduncular groove; in 

 O. celatus and 0. crassus, however, they are removed from the latter and converge posteriorly. 



The form of the shells of 0. acuminatum shows but slight deviations. In some specimens the highly arched beak 

 of the large shell, which falls oft steeply posteriorly and laterally, is somewhat more sharply pointed and the tip of the 

 beak seems to be compressed toward the side. In others the S-shaped section of the border of the large shell is more 

 slightly wavy, but in the differences of form no rule can be perceived. It is similar with the small shells. 



In regard to regularity, however, the outer surface of the shell seems to be subject to greater deviations than the 

 foregoing species in their concentric striations. Besides forms with entirely smooth and lustrous shell surfaces, 

 some occur that have very irregularly deep sculptured concentric furrows, which become especially irregular toward 

 the anterior border and give the shells a dull aspect. 



The area is in both shells very unevenly striate, and in the large shells, on account of the fracture in the pedun- 

 cular furrow and the somewhat incurved tip of the beak, it is, as a rule, slightly excavated. With normally built shells 

 the traces of the pseudo-area show slight inclination to separate themselves from the borders of the peduncular fur- 

 row; and the deviations (which some specimens pointing to the variety alatus show in this direction) are always very 

 small. In the small shell, whose traces of a pseudo-area lie opposite those of the large shell, these deviations show 

 themselves in a greater breadth of the furrow similar to the peduncular groovelike furrow which forms the splanch- 

 noccelic part of the area. 



The crowding of the splanchnoccele into the beak of the large shell (which is conditioned by the high arching 

 and the uniformly slight thickening of the same) is only a more marked form of the groove-shaped excavation of the 

 base of the area. Shells whose beaks are more flatly arched show as a rule a more distinct thickening of the posterior 

 part, and, in connection therewith, an unexcavated, massive tip of the beak. The same is also the case with the small 

 shells of this species. Likewise the bridge supporting the peduncular furrow is foreshadowed in that the groovelike 

 hollowing out of the base of the area at the mouth of the peduncular furrow is transplanted into the splanchnoccelic part. 

 The inner configuration of the shells is, on account of the defective thickening, as a rule slightly pronounced. An 

 exception is formed by the spots of attachment of the central muscles of both shells and the median swelling of the 

 small shell, which are always easily perceptible. The former are shown deeply sunken in the large shell, and in the 

 roundish form characteristic for the Schmidtias, while the laterally situated places of attachment of the outside lateral 

 muscles are only slightly deepened, but are separated from those by a distinct ridge. In a small shell the places of 

 attachment of the central muscles appear in the form of strong protuberances. The median swelling is, in opposition 

 to that of 0. obtusus, at its anterior border (between the places of attachment of the anterior lateral muscles) undivided, 

 and posteriorly united with the feeble median septum. In a large shell the latter appears as a very sharp, small ridge. 



