94 SYNOPSIS OF AMERICAN FOSSIL BKACHIOPODA. [nnxST. 



The {'xtenial characters as ex])re88eil by both valves are niaiiifestly nearer to Kntor- 

 giua than to any telotremate geuus. » * * It should bo noted, however, that the 

 young of ChoueteSjProductus, Stropheodonta, Orthothetes, Leptiena, Plectambonitcs, 

 and Strojihomcna, all have little or no indication of a straight hinge line, and that 

 the extension of this member takes place during later nealogic and ephcljolic growth. 

 (Pp. 150-151.) 



By far the greatest number of Neotremata occurring in the Lower 

 Cambrian are species of the family Acrotreticbe. To the writer it has 

 always seemed strange to suppose that this family has been derived 

 through the Trematidfe, but the above interpretation of the prodeltid 

 ium in Discinisca indicates that the turning of this j)]ate i^osterior to 

 the j)edicle at once led to holoperipheral growth in some of these early 

 forms. In some species of the Acrotretida> there is a true deitidium. 

 In Acrothele the cardinal area is flat, Avithout any trace of a deitid- 

 ium, whereas in Acrotreta and Conotreta, which have high cardinal 

 areas, there is a narrow concave depression bisecting it. These del- 

 tidia, whether convex or concave, are in all probability initiated by the 

 prodeltidium, as in the Protremata. In the family Trematidje there 

 appears to be nothing homologous with the deitidium, since the plates 

 situated in the apex of the wide triangular fissure of Schizocrauia and 

 Lingulodiscina seem to be formed anterior to the pedicle and subse- 

 quent to its movement posteriorly with growth, and not posterior to 

 the pedicle, as in the Acrotretidse. These plates in the Trematidie 

 should i^robably be homologized with the listrium of the Discinidie. 



The complete harmony of the muscular system in the Protremata 

 and Telotremata is no evidence in itself that the latter were derived 

 from the former. The occurrence at the base of the Cambrian of very 

 primitive species of the four brachiopod orders is proof that diver- 

 gence took place very early in the history of the class, and while there 

 is little knowledge of the muscles in either Iphidea, Kutorgiua, or Pro- 

 torhyncha (P. f minor and P. f ambign a), the earliest genera of Atremata, 

 Protremata, and Telotremata, respectively, there is some evidence for 

 supposing them to be as in the type embryo stage of living species. 

 The high degree of specialization attained by Lingula (Atremata), as 

 exem[)lified by the burrowing habit, long peduncle, and absence of valve 

 articulation, is the cause for their complex muscular system, while the 

 development of a functional hinge in the Protremata and Telotremata 

 has led to the retention of very primitive conditions or to the simiiliti- 

 cation and harmony of the muscles throughout these two orders. 



The presence of a terminal intestinal opening in the living species of 

 the Atremata and ISTeotremata and its general absence in those of the 

 Protremata and Telotremata is no longer held to have phylogenetic 

 significance, as many of the Paleozoic species of the two latter orders 

 afford good evidence of such having been present in the median line as 

 in living Crania.^ 



1 See p. 113. 



