OBOLIDiE. 445 



into a depression; peduncular furrow shallow but plainly developed, slightly diminished toward the tip of the beak. 

 Traces of a pseudo-area bent slightly concave to the area lamellse at the median line of shell, converging into the 

 tip of the beak. Shells almost uniformly thick, toward the beak only a little thickened. Traces of the principal 

 vessels broad, shallow, little prominent; middle swelling and lateral furrows of the same scarcely projecting. Secondary, 

 inwardly radiating vessel traces in the large shell in the principal direction perpendicular to the principal vessels; 

 in the small one curved anteriorly, in both very indistinctly ramified. Central groove of large shell pestle-shaped, 

 with a slight swell. Apex of central groove relatively broad, somewhat drawn out, parallel-bordered, generally some- 

 what involved with the combined central places of attachment of the muscles. Median swelling of small shell dis- 

 tinctly developed, growing over into the median septum, at times separated by a fine median groove. Corneous 

 processes and sinus receding very much. 



Observations: 0. celatus varies as all Obolus species in its external form and internal configuration of its shells. 

 The want of space, however, forbade the illustration of these alterations in a manner similar to that used for the prin- 

 cipal species of Eichwald's genus. I had to restrict myself to the illustration of the extreme members of the series of 

 forms. This remark concerns all species of the subgenus Schmidtia. 



Aside fi-om the form figured by Volborth, some with circular outline and others with flattened anterior border 

 occur, which are connected with the principal forms by transitions and were demonstrated in their extreme members 

 as 0. celatus orbiculatus and 0. celatus prxcisus. To all members of this group, brought together as 0. celatus, a note- 

 worthy peculiarity of accessory nature is common which is lacking in the remaining species of the subgenus Schmidtia, 

 and which, therefore, may be looked upon as a further argument for the cospecificity of the forms mentioned. This 

 peculiarity is a fine netlike coating, which may be found on many of these shells and which often covers the surface 

 of the shell in several superimposed layers in the form of crusts. It may indicate the existence of a bryozoan. 

 Traces of the netlike coating are, however, also found in the specimens of the variety pi-s^'cisus. 



The tip of the beak and the anterior border of the large shell de\'iate in some specimens fi'om the normal character 

 in such a manner that both recede somewhat from the level of the border of the shell; then in the corresponding small 

 shell the reverse occurs so that the closing of the shell is secm-ed. The receding of the beak is nevertheless never so 

 strong as, for example, in 0. obtusus. The flat plate-shaped border [Volborth, 1869, p. 210] of the small shell is formed 

 "by the last separated lamella of the border, which is marked oft from the remaining surface by the principal vessel 

 furrows which com-se along its inner border, and, in consequence of the flatness of the small shell, it stands out more 

 distinctly. In the large shell the principal vessel fm'rowsare moved more toward the interior of the splanchnocosle, 

 and as the shell also has a stronger arching, the lamellae of the border recede farther. Besides, this formation, which 

 is found in all species of the subgenus Schmidtia, is very variable in its strength and in some small shells hardly 

 perceptible. On the other hand, it at times also occurs in the large shells in which the peripheral canals play the same 

 part as the principal vessel furrows. 



The surface of the area of the large shell in this species of the subgenus Schmidtia is least broken in the peduncular 

 furrow, and the area itself reminds one of that of the typical species, especially in that the traces of the pseudo-area 

 of the area lamellae converge into the tip of the beak. The pleurocrelic parts of the area of both shells are only 

 slightly prolonged into the lateraJ borders. On the other hand, the area of the small shell deviates from that of 0. 

 ■apollinis by the deepening of its splanchnocoelic part, which possesses in consequence of the special position of the 

 traces of the pseudo-area in this species a subtriangular form. 



The form of the central groove varies within rather wide limits. Through lateral restraint of the posterior part of 

 the protruded parallel-bordered tip there results an hregular biscuit-shaped outline, and through compression of the 

 posterior broad part of the central groove in the median direction, a simultaneous shortening and tapering of the anterior 

 part. Some shells show the protruded tip laterally compressed, so that only a narrow groove is left, which is then as a 

 rule inclosed by strong swellings. In others this groove possesses a breadth which almost equals the posterior part of 

 the central groove. All these alterations of form, which seem subject to no law, are also peculiar to all other species 

 of the subgenus. 



The side septa and median septum are, even if slightly, developed in both shells. The former as a rule stand 

 out more distinctly in the large shells; the latter, however, in the small ones, where it is more distinctly expressed, 

 appears as a prolongation of the always strongly pronounced median swelling. 



The corneous processes and the sinus of the small shell are rarely to be distinguished. Yet in some specimens, 

 in reflected light under a lens, two fine ridges are seen, which converge from the external sides of the places of attach- 

 ment of the central muscles toward the beak and, with the median swelling, form a triradiate figure. 



In spite of their minuteness the places of attachment usually stand out distinctly, especially those of the central 

 muscles, which in both shells as a rule are let down deeply with swelled borders. Also the separation of the combined 

 places of attachment is attained with the help of a lens, although naturally with more difficulty than in the larger forms 

 of Eichwald'sgenus. It is very difiicult to perceive the places of attachment of the transmedian and the combined places 

 of attachment of the outside and middle lateral muscles in the small shells. The places of attachment of the central 

 muscles, as also those of the outside and middle lateral muscles of the large shell, have more roundish forms and are 

 not so closely connected as in the species of the subgenus Euobolus. Noteworthy is also the somewhat deviating 

 position of the places of attachment of the central and anterior lateral muscles in the small shell. 



The direction of the longitudinal axes converge, in the former toward the beak, in the latter toward the anterior 

 border. As these places of attachment have in by far the most cases a pronounced elongate form, this position is very 

 striking and was also noticed by former observers [Volborth, 1869, p. 210]. 



