394 STUDIES IN EVOLUTION 



ventral valve in the Rhynchonellae, Terebratulse, etc., as a 

 specialized extension of the valve for the protrusion of the 

 pedicle. (Compare the extreme development of the umbo 

 in the genus Terebrirostra.) That these parts are also homo- 

 logues, it is difficult to prove, on account of the pedicle- 

 sheath becoming more degenerate as maturity approaches ; 

 but, assuming this homology, the sheath and its gradual dis- 

 appearance may be regarded as an indication of degeneracy in 

 the family, the presence of the sheath pointing toward a 

 derivation from the rostrate type. 



The atrophy of an organ so highly specialized as the sheath 

 is, aside from any consideration of relationship to other 

 groups of the brachiopods, itself confirmatory of such de- 

 generacy. Furthermore, it will be noticed that there is, 

 throughout these strophomenoids, an inclination, as mature 

 growth comes on, toward the simple triangular pedicle- 

 apertures in Orthis. The disappearance of the pedicle-sheath 

 leaves the aperture of the ventral valve essentially free, as 

 seen in Leptcena rhomboidalis and Orthothetes subplanus, 

 while the aperture of the dorsal valve is filling pari passu 

 with a callosity. In other words, the structure of these 

 parts is actually degenerating toward maturity, to that of 

 Orthis, which is the simplest, least differentiated condition 

 among the articulated brachiopods, and serves to fortify the 

 position of the genus at the base of the entire series. In 

 Orthis, the pedicle-apertures on both valves are of the same 

 size in early growth, and have undoubtedly acted together 

 as a single opening, through which the fleshy arm was pro- 

 truded as much on one side as the other, a fact indicative of 

 an extreme lack of differentiation in the two valves in the 

 articulate species, but agreeing closely with some of the 

 inarticulate genera, as Lingula, Leptobolus, Obolus. The spe- 

 cialization which accompanies subsequent growth confines 

 the pedicle more closely to the ventral aperture, and, as a 

 result, the dorsal aperture is gradually filled by a callosity. 

 Thus, also, the Strophomenidae ; but Orthothetes subplanus 

 shows at maturity what has not yet appeared in Dalmanella; 



