440 CAMBRIAN BRACHIOPODA. 



animal. The large shell of a youthful specimen is not essentially distinguishable in the general arrangement of the 

 characters from the typical species, unless stress be laid on the extraordinarily strongly shortened splanchnocoele. In 

 one specimen a slight swelling is already noticeable, which, however, does not extend to the splanchnocoele, but, 

 moved from the places of attachment of the muscle far to the front, lies in the brachioeoele. Another already shows 

 the beginning of the formation of terraces in the form of two slight obliquely placed swellings, which are connected in 

 advance growth and widen out to a surface whose border (which faces the beak and falls off perpendicularly) shows 

 an inlet corresponding to the protruded tip of the central groove in the median line of the shell. In further growth 

 of the shells one terrace is built upon the other, each following the other scalariformly, receding from the border of 

 the preceding one and thus gradually forming the mass. This thickening of the shell is extended only on the central 

 part of the shell. At both sides deep, broad furrows remain, which are retained for the principal vessels. Most likely 

 the mass is gradually flattened toward the anterior border. Unfortunately, all specimens are injured in such a manner 

 that nothing definite could be ascertained. 



With the small shell the thickening takes place in entirely similar manner. Here the terraces are chiefly separated 

 by the lateral angular places of the brachioeoele, whic'h are formed by the protruding splanchnocoele and form a mass 

 scalariformly sloping, which cuts deep into the protruding splanchnocoele. A youthful specimen of the small shell 

 shows a strongly developed mass, which, however, was for the greater part destroyed by the treatment with dilute 

 hydrochloric acid. This manner of preparation was chosen to lay bare the posterior parts under the deeply hollowed 

 umbonal border, which was also satisfactorily accomplished at the expense of the strongly weathered lamelte of 

 thickening. 



Of the characters whose development is connected with the thickening of the posterior part of the shell, as the 

 septa, corneous processes, and heart-shaped groove, nothing is to be discovered in the adult shells of 0. antiquissimus. 

 In young specimens, however, the central groove is mostly distinguishable, even if it is always very small, flat, and 

 indefinitely bordered. 



The principal vessels are sunk in the small shells in the usual manner, but are not divided by a middle swell- 

 ing. With the adult they run in deep furrows at both sides of the central mass. Their course can not be followed in 

 the anterior half of the shell on account of the injured condition of the shells. The accessory vessels and the peripheral 

 canal are only perceptible in indistinct traces. The former seem to run without branching, and perpendicularly to the 

 principal vessels. 



The places of attachment of the muscles of 0. antiquissimus have in general the same arrangement as those of the 

 typical species. In the latter those which lie more toward the center of the shell and toward the anterior are, 

 however, corresponding to the disposition of the splanchnocoele, strikingly long drawn out. 



The peduncular muscle and the divided umbonal muscle are attached in the young shells exactly in the same 

 manner and at the same place on the bottom of the shell of the ventral valve as in the other species. In further 

 growth of the shells small elevations are formed at th'e places of attachment. The progressing development of these 

 elevations may be followed in a series of shells. The center one of these elevations is gradually altered through corre- 

 sponding attachment of the lamellae into a toothlike process, which reaches in front of the peduncular groove freely 

 into the splanchnoccele and serves the peduncular muscle as a place of attachment. The places of attachment of the 

 divided umbonal muscle have gradually moved to the rimlike border of the shell (close to the peduncular furrow), 

 whose narrow sides are turned inward. The combined spots of attachment of the transmedian and anterior lateral 

 muscles of the large shell undergo the same change of place. 



The case is entirely similar with the places of attachment of the small shell, which lie at the base of the area, and 

 the pleurocoele (umbonal, transmedian, outside lateral, and middle lateral muscles). They all move, in the adult indi- 

 viduals, into a place corresponding to their position in the youthful shell, to the narrow side of the rimlike border 

 which surrounds the posterior part of the shell. The place of attachment of the umbonal muscle acquires, aside from 

 this, a special broadening in the form of a pier-shaped platform, which protrudes into the inside of the shell. The 

 pierlike construction gives this place of attachment of the umbonal muscle, which (judging by the mutual attrition of 

 the beaks) displays a considerable strength, a special firmness. 



All these places of attachment, which lie at the border of the shell, are grained like shagreen, coffee-brown colored, 

 and covered with small, lustrous, dark-brown knots. 



More important than those just described is (in comparison with the typical species) the dislocation of the places 

 of attachment, lying in the interior of the shell. They all are, in consequence of the extraordinary shortening of the 

 splanchnocoele of this species, pushed back so far toward the beak that (especially with the small shell) an entirely 

 distorted figure of the typical figure is brought about. The position of the combined central places of attachment of 

 the large shell is in this regard less striking. The latter are, to be sure, moved so near to the area that only a small 

 triangular space is left for the central groove, but the difference between this arrangement and that with 0. cdatus 

 orbiculatus is, however, proportionately slight. Of greater importance is the aberrancy in the opposed position of the 

 individual spot of attachment of this complex, in which, as already often mentioned, the places of attachment of the 

 central muscles are arranged behind those of the outside lateral muscles. The combined central places of attachment 

 could not be demonstrated in the only adult specimen of the large shell; they are lost in the strongly developed 

 configuration of the inner surface. 



The small shell, on the other hand, presents an entirely singular view. The strongly developed places of attach- 

 ment of the central muscles are, with the young shells, pushed back to the most posterior part of the splanchnoccele, 

 and lie under the platform which serves the umbonal muscles as a place of attachment, and which covers the same. 



