OBOLID^. 439 



tip of the central groove. Both places of attachment narrow, long drawn out, and placed transversely to the shell. 

 On the facing ends of the places of attachment of the outside lateral muscles and divided by the projecting tip of the 

 central groove the small, round, quite deeply depressed places of attachment of the middle lateral muscles. Places of 

 attachment of the central muscles in the small shell moved far into the excavated tip of the beak, lying under the 

 platform which serves the umbonal muscle as a place of attachment; very much prolonged, divided from one another 

 by the median swelling. Places of attachment of the anterior lateral muscles of the small shell likewise moved very 

 far toward the posterior, Ijdng about in the posterior third of the shell, divided by a fine furrow at the end of the 

 swelling. 



Parietal band moved away from the places of attachment in its anterior part, coursing at the border of the scalari- 

 formed terraces of thickening. 



Observations: The immature shells of this extraordinary species show unmistakably the characters of the genua 

 Obolus, while these latter are not so easily distinguished in adult individuals. This is especially the case with the 

 characters of the inner surfaces of the shell, which acquire an entirely altered appearance in consequence of the 

 formation of the shell, which varies in age, and the altered disposition of the thickening of the shell. 



The relative height of the adult shell is larger than that of the young. In the same mentioned specimens it amounts 

 to 0:171; 0:307; 0:262. The falling off of the arching to the tip of the beak is very flat with the young shell; with 

 the old, as a rule, perpendicular. However, essential aberrances take place with the adult individuals; some show 

 the falling off of the beak considerably flattened. 



As with all other ornamented species, the ornamentation of the outer surface of the shell is altered with the advanc- 

 ing age of the animal. The delicate concentric striation has no waves until the close of the flrst vegetative period and 

 reminds one with its parallelism (in a strict sense) of that of 0. silurieus. At times, however, it shows some irregu- 

 larities, as the parallel uniformly curved ribs at times flow together or become disturbed. With the beginning of the 

 second vegetative period the waves occur, which, however, are often disturbed inform, and are resolved into separate 

 small swellings. This ornamentation Volborth has fitly named ".undulate-humped." Toward the anterior border 

 the swellings are crowded again, the waves become smoother, and the concentric ribs flatten, so that the ornamentation 

 is almost lost close to the anterior border. 



The construction of . antiquissimus, in comparison with the other Oboli, is somewhat negligent, if we may use that 

 term. The growth lamellae lie very irregularly on one another and protrude very unequally with their anterior sharp 

 borders; at the same time the coherency of the lamellae is so loose that they fall apart at the least occasion. The 

 irregularity of the construction of the shell is shown principally at the posterior borders, at which the lamellse are 

 displaced perpendicularly under one another, and on account of unequal stratification give the appearance of the leaves 

 of an uncut book. 



The individual rings of the growth lamellse (taking the latter as a whole, not only the visible parts on the surface 

 of the shell) are very massive with 0. antiquissimus, and are club-shaped toward the beak, in the profile, as with 

 other species. But while in the remaining species they at the same time become narrower (crescent-shaped) toward 

 the beak, they here retain their breadth to the traces of the pseudo-area of the area lamellae and then pass over (in 

 the splanchnocoelic part of the area of the base) into a position perpendicular to the plane of the border of the shell. 

 The breadth of the rings (surface of displacement) increases with the advancing age of the shell, independently of 

 whether the existing vegetative period produces a thin or thick lamella. The thickness is understood as the perpen- 

 dicular distance between the sm'faces of separation. 



According to this, the construction of the shell takes place as follows: At the anterior border the growth lamellse 

 are separated in ordinary manner, each one sticking halfway under the preceding one. Toward the posterior the 

 individual rings are pushed more and more below one another until they lie perpendicularly below one another at 

 the posterior borders of the shell. Now, as every new ring surpasses the preceding in breadth, and the splanchno- 

 ccele is nominally reduced in thickness, the rings, which lie perpendicularly upon one another, form a very mas- 

 sively hollowed border, similar to a rim, at the posterior borders of the shell, which is also continued even in the 

 beak. There, however (between the traces of the pseudo-area), they are built up by the lamellse of the splanchno- 

 coelic part of the area, which lie in front of one another, perpendicular to the plane of the border of the shell. In 

 the umbonal part of a small shell the described lamellse (which lie over each other) of the posterior lateral borders, 

 as well as the posterior' border of the shell which is formed by the splanchnocoelic area lamellas, ihay be seen 

 distinctly. 



A principal distinction between the construction of the shell of 0. antiquissimus and that of the other species is 

 not present. The strengthening of the principal part of the shell (which incloses the visceral cavity) takes place with 

 the first named through strengthening of the outer lamellae at the expense of the inner thickening, but with the other 

 species through strengthening of the inner thickening at the expense of the outer lamellse. 



A noteworthy appearance, which has only been noticed with 0. antiquissimus, deserves to be emphasized. The 

 ring-shaped lamellse of the anterior part of the shell consist of rock-mass (limestone), but are, as all lamellae, clothed 

 with a homogeneous (corneous) layer, so that it gives the impression that the individual lamellas had been hollow 

 and after the death of the animal had been filled with calcareous mud. It is, however, more likely that the inner 

 layer of the canal, which is less capable of resistance, was dissolved after the death of the animal and restored by the 

 penetrating calcareous mud. 



In still greater measure than through the just described formation of the border the configuration of the inner surface 

 of the shell becomes altered through the disposition of the lamellse of thickening, which deviates iii the age of the 



