SILURIAN BRACHIOPODA 53 



inception of the shell. The test is flat, both valves being shallow and 

 depressed toward the anterior margin ; the ventral beak high and erect, 

 the dorsal beak inconspicuous and rounded. The foramen, which is 

 undoubtedly triangular in the initial shell, has, at this stage, its basal angles 

 slightly rounded by the faintly developed deltidial plates. The plications 

 are six in number on the ventral, and five on the dorsal valve, the middle 

 one of the latter not reaching so far toward the beak as those adjoining it, 

 and toward the anterior margin being depressed below the lateral portions 

 of the shell. General outline subcircular or subpentagonal, as in the full- 

 grown shell. 



DEVELOPMENTAL VARIATIONS 



General Form and Outline. Embryos of less than 3 mm. in length 

 are more nearly circular in outline than at any subsequent period of the 

 existence of the individual. Directly thereafter, the hinge-line represents 

 the greatest diameter of the shell, and the outline becomes subpentagonal, a 

 feature which is more apparent in young individuals having between 3 

 and 10 mm. length, as the increasing rotundity of the shell with the 

 approach of maturity has a tendency to obscure, in a measure, this out- 

 line. At the earliest stage studied, the dorsal valve is distinctly depressed 

 along the median line, forming a sinus containing a single plication which 

 does not reach to the beak (plate 6, fig. 14^). This sinus gradually 

 becomes shallower, and the plications are increased by intercalation until 

 they are three in number (fig. 14^). In the next stage, all evidence of a 

 sinus upon the anterior margin disappears, leaving it even and straight 

 as shown in figure \^e ; then the anterior edge becomes reflexed, showing, in 

 subsequent stages of growth, a fold where there had previously been a 

 sinus, this fold bearing at first three, then five, and eventually, in the 

 mature individual, seven plications (fig. i^d,c,f). This very remarkable 

 reversion of the fold and sinus relatively to the valves which bear them, is 

 also seen in the species Rhynchotreta cuneata and, in all adult specimens, 

 may be clearly traced upon the earlier or embryonal portions of the valves. 



