824 PALEONTOLOGY OF NEW YORK. 



Ill ACROTRETA, CONOTRETA, LiNNARSSONIA, AcROTHELE and IPHIUEA the pedicle- 



aperture is persistently located at the apex of the pedicle-valve. This group of 

 genera is one of very early date, for the most part contemporaneous with Paterina, 

 and the existing evidence would indicate that it was not directly ancestral to the 

 line of Trem atis-Orbiculoidea (Discmw^). The incipient formation of an inter- 

 nal foraminal tube is seen in several of these genera (Acrotreta, Acrothele, 

 Linxaussonia), and this feature attains its maximum in the true Sipiionotreta 

 of the Lower Silurian, where the foramen is still apical and the tube wholly 

 internal. Hence Sirno.voTRETA appears to be a normal termination of this line 

 of descent. Schizambon, in the comprehensive meaning of the term ascribed 

 to it in this work, has the pedicle-passage superficial, and in such shells as 

 Schizambon fissus, Kutorga, and var. Canadensis, Ami, the condition of this 

 passage is perfectly analogous to that of Siphonotreta, the entire difference 

 being in the enclosure of the latter. In Schizambon the fibers of the pedicle, 

 extending through the foramen near the middle of the pedicle-valve, were 

 directed toward the apex of that valve, and along the concave floor of the 

 external pedicle-groove. The inner aperture of the pedicle-tube in Siphono- 

 treta, corresponds to the " foramen " of Schizambon, and the outer aperture 

 or true foramen of the former to the grooved umbo of the pedicle-valve in the 

 latter. Hence in Schizambon, thus considered, there is no evidence of a prog- 

 ress of the external aperture, or true foramen, anteriorly beyond the apex of 

 the pedicle- valve. These two genera are but slight departures from the same 

 type of structure, but it would appear that this deviation took place during 

 primordial times, as the typical Schizambon (S. tijpicalis, Walcott) is a primordial 

 fossil. The newly described genus, Trematobolus, Matthew* (T. insignis, Mat- 

 thew, typej, appears to be another primordial representative of this structure, 

 with the tubular enclosure of the pedicle more highly developed. Thus all 

 these genera, from Acrothele to Schizambon and Siphonotreta, possess an 

 apical foramen, and the development both of the internal tube and the 

 corresponding extennal groove has been a gradual one. They represent 

 termini of slightly divergent series ; consequently they may all be safely 



* Canadian Reconl of Science, January, 1893, pp. 277-279, figs. I a-d. 



