METAZOA VERMES. 275 



The changes which take place in the development of a 

 Protrematous shell are well shown by Bilobites varicus 

 Conrad (PI. 673, figs. 1-12). It begins as a smooth, 

 straight-hinged form with the hinge as broad as the shell 

 and with a rounded front margin (figs. 1,2). Occasion- 

 ally while the shell is in the nepionic stage the deltidium 

 of the pedicle valve is retained, as seen in fig. 3, which 

 also shows the cardinal area and pedicle opening. The 

 deltidium is lost in the neanic or the ephebic stage and the 

 pedicle passes out of the large opening. Gradually the 

 rounded front margin, seen in figs. 1,2, becomes straight 

 (fig. 4), and later a sinus is developed (fig. 5) which 

 afterward becomes more pronounced (figs. 6-n). 



The pedicle valve is at first shorter than the brachial, 

 and for this reason is not seen in figs. 1,2, 4-6 ; soon, 

 however, it grows longer (fig. 7) and the cardinal area 

 with its pedicle opening is distinctly seen (figs. 8-10). 

 The adult shell (fig. n) has a narrow hinge line, while 

 the anterior portion of the shell is broad and strongly 

 bilobed. In the old age or gerontic stage the sinus 

 becomes nearly obliterated (fig. 12). and the anterior 

 portion of the shell resembles that of the young stage 

 represented in fig. 5. 



One of the most differentiated members of the Protre- 

 mata is the genus Pentamerus (No. 674, P. oblongus 

 Sow.). The variation in form of this genus is shown by 

 the specimens (No. 674, e, f) which are casts of the 

 interior. The beaks even in the young stages (No. 673, 

 a-d) are incurved, and so strongly is this the case in the 

 adult (e, f) that the small pedicle opening is concealed. 

 The deltidium when present (which is rarely the case) is 

 concave. 



This genus is an exception among the Protremata, inas- 

 much as the arms are supported by two limy processes 

 called crura. These crura unite to form a plate, the cru- 

 rulium, which is used for the attachment of muscles. 

 The pedicle valve also has a large, strong, internal plate, 



