388 STUDIES IN EVOLUTION 



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, pro- 

 ducing a consequent incurving of the beak over the cardinal 

 area ; as in Dalmanella and CamarotoecTiia indianensis. Some 

 species present both beaks as incurved, a condition well 

 represented in Meristina Maria, Dictyonella reticulata, Cam- 

 arotoechia acinus, and C. neglecta. In Anastrophia the com- 

 parative relations of the valves become reversed from their 

 initial condition, on account of the more rapid increase 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, with a corresponding fold in the ventral, begin- 

 ning soon after the initial stage of the shell has been passed, 

 and disappearing before the shell is half grown. Those forms 

 presenting this feature to a marked degree are Rhynchotreta 

 cuneata, Oamarotoechia Whitii, C. indianensis, C. neglecta, 

 O. acinus, Atrypa reticularis, and Homceospira sobrina. In 

 Rhynchotreta cuneata and Atrypa reticularis (Plate XVIII, 

 figures 12-15, and Plate XX, figures 12-14), the gradual in- 

 ception of this sinus, its maximum development and obsoles- 

 cence, and, finally, its reversion into a fold which thereafter 

 persists and usually increases in prominence in all the suc- 

 ceeding stages of growth, have been shown. The embryonal 

 sinus is not present in Spirifer, Anastrophia, Dictyonella, 

 Meristina, Whitfieldella, Dalmanella, Rhipidomella, nor in the 

 StrophomenidaB. Such of these as show a dorsal fold or ven- 



