SILURIAN BRACHIOPODA 85 



for such forms as the Strophomenidae, which vary in convexity, either 

 very slowly or not at all, up to individuals about half-grown, when the 

 valves become more or less deflected and often concave. Such a re- 

 version in the shell is in conformity with the degeneracy which is traced 

 in the development of the cardinal area and pedicle-sheath, mentioned 

 on a subsequent page. 



The observations of BROOKS and MORSE, in the works just cited, 

 show, that in both the hingeless and the hinged brachiopods, as repre- 

 sented by Lingula and Terebratulina, the early stages of the shell 

 approach a subcircular outline, and BROOKS remarks (pp. cit. p. 43), that 

 "the recent and fossil shells of the various species of Crania, Lingula, 

 Lingulella and Obolus, and other hingeless Brachiopods, furnish a series 

 of adult forms representing all the changes through which the outline 

 of Lingula pyramidata passes during its development." 



In these respects, then, uniformity is established in the embryology 

 of the ancient Silurian types and their modern descendants. 



VALVES. The dorsal valve in young shells is smaller than the 

 opposite, and usually more depressed. These relations, as a rule, are 

 continued up to adult size, except that the ventral valve often increases 

 more rapidly in convexity, producing a consequent incurving of the beak 

 over the cardinal area, as in Orthis and Rhynchonella indianenns. Some 

 species present both beaks as incurved, a condition well represented in 

 Whitfieldia maria, Eichwaldia reticulata, Rhynchonella acinus, and R. 

 neglccta. In Anastrophia, the comparative relations of the valves become 

 reversed from their initial condition, on account of the more rapid in- 

 crease in the depth of the dorsal valve, so that, at maturity, the dorsal 

 beak is much incurved, and often the umbo extends beyond that of the 

 other valve, although the beaks preserve their normal condition of 

 superposition. 



Several of the species show an embryonal sinus in the dorsal valve, 



