OBOLID^. 447 



Principal vessel furrows broad and flat. Central swelling of same slightly pronounced. Central swelling of the 

 peripheral canals mostly separated into square sections by furrows which are parallel, directed obliquely outward, 

 and which subside posteriorly. Secondary inwardly radiating vessel furrows strongly and iiTegularly branched; in 

 the large shell perpendicular in their principal dii-ection to the principal vascular canals; the posterior ones running 

 out into the interior of the brachioccele, those more anteriorly placed bending toward the peripheral canal and 

 opening into the same. In the small shell these furrows take an oblique direction, anteriorly and internally from 

 the lateral parts of the principal vascular canals but posteriorly and externally from the parts turned off toward the 

 places of attachment of the anterior lateral muscles. Secondary outwardly radiating vascular traces in both shells 

 rectilinear, perpendicular to the principal vascular furrows, plainly forked, close before the opening into the 

 peripheral canal. 



Places of attachment of the muscles in both shells strongly sunken, those of the central and outside lateral muscles 

 of the large shell oval, somewhat separated from one another by a small swelling. 



Parietal band in both shells plainly developed, normally running out, brought out through the fine-grained condi- 

 tion of the surface . 



Observations: 0. crassus reminds one, in the form of its shell, of 0. acuminatus in relation to the position of the 

 traces of the pseudo-area; it takes, on the contrary, an intermediate position between 0. celatus and 0. obtusus. The 

 relations to 0. acuminatus have abeady been mentioned (p. 443). Further, at the same place it was pointed out 

 that the traces of the pseudo-area of both species (0. crassus and 0. celatus) converge posteriorly. The difference in 

 the position of the converging pseudo-areas of the two last-named species is, however, very essential and consists in the 

 fact that in 0. celatus the same opens into the tip of the beak; with 0. crassus, on the contrary, the borders of the area 

 intersect at a certain distance from the beak. The last can, under some circumstances, become so large that the traces 

 of the pseudo-area maintain a subparallel position which reminds one of that in 0. obtusus. A confounding of the two 

 species in this rarely occurring case is meanwhile excluded on account of the high triangular area and the deviating 

 form of the shell of 0. crassus. In a majority of the shells the convergence of the traces of the pseudo-area is dis- 

 tinctly pronounced. 



The high, coarsely striate area has a plump form in consequence of the pleurocoelic part being only slightly drawn out 

 into the lateral borders, and on account of its convex external borders, which contrast strongly with those of 0. celatus. Its 

 base runs almost straight, is always composed, in common with the other species of the genus Obolus, of three shallow 

 curves, and is slightly grooved; tip in the large shell usually recedes somewhat from the plane of the shell borders. 

 The break in the surface of the area in the large shell, in the peduncular, is insignificant but somewhat stronger than 

 in 0. celatus; the sunken splanchnoccelic part of the area in the small shell has a trapezoidal form, in consequence of 

 the special position of the traces of the pseudo-area, and is somewhat hollowed out in the median direction. * * *■ 



The profile of the shell borders shows in both shells, as a rule, a straight line; in the small shell it is at times 

 slightly bowed concavely. The shell borders themselves are, corresponding to the thick condition of the shells of 

 this species, thicker and less sharp than with the other species of the subgenus Schmidtia. 



The external surface of the shell possesses a rather coarse concentric striation, which becomes somewhat stronger 

 toward the anterior border, but does not stand out in the projecting intricate form as observed in other species of 

 the subgenus. 



The character of the internal surface of the shell offers in general the same picture which we have learned to know 

 in 0. celatus. With the majority of the shells the central pit has the normal, pestle-shaped form, and is inclosed by 

 a more or less pronounced swelling. One specimen shows the latter at the posterior border and on the sides of the 

 depression swollen up. With another specimen it is, on the contrary, entirely absent and the central depression appears 

 shallowed and indefinitely bordered. This shell is in a somewhat abnormal condition. It shows in profile an arrest 

 of growth through which the greatest height is shoved toward the beak. At the same time it shows the places of 

 attachment of the central muscles as thick protuberances — especially on the right side. The septa of both shells are 

 very weakly developed, likewise the corneous processes of the small shell, which, as with all Schmidtias, recede before 

 the powerfully developed median swelling. The latter is in its anterior half provided with a fine median groove. 



In reference to the places of attachment of the muscles of 0. crassus there is nothing essential to be added to the 

 condition found in the other Schmidtias. They are found here in exactly the same position and condition as in the 

 other species of the subgenus, so far as the modified form of the shells does not condition slight alterations. It is at 

 all events to be insisted upon that these characters in the present species are found in a condition of distinctness, which 

 the remaining species of the subgenus do not show in equal degree. 



Into the circulatory system of 0. crassus, on the contrary — which, through its prominent development and preser- 

 vation, explains many conditions which, in the other species, must remain undecided — I shall enter more closely. 



The principal vessels of the large shell of this species are moved somewhat nearer to the median line than in 0. 

 celatus and at tinies acquire a subparallel position. This character, nevertheless, is extremely variable and is dependent 

 on the form of outline of the shell. Broader forms show these traces more approximated to the border of the shell. 

 In the small shell the principal vessel furrows run in the manner known near the border of the shell and parallel to 

 the same. The opening of the principal vascular canals of the large shell into the peripheral canal is plainly to be 

 seen under a lens by reflected light. * * * 



The peripheral canals show a peculiar formation, which hitherto has not been noticed in the other species of the 

 subgenus Schmidtia. The median swelling of the peripheral canals (especially at its posterior parts) is separated by 

 short, obliquely placed cross furrows, which anteriorly run inward, posteriorly outward, into a series of small square 

 sections, which as a whole resemble a twisted ribbon. This character is somewhat inconstant and seems to appear 



