SILURIAN BRACHIOPODA 91 



the Strophomenidee. In the latter forms, we have seen that the remarkable 

 development of the pedicle-sheath is primary, and is invariably more or less 

 atrophied with age, and probably functionally inactive at maturity. Hence 

 the retention of this sheath in any species at maturity is the perdurance of 

 what must serve as an embryonic character within the limits of this family. 

 It cannot escape observation that the pedicle-sheath is in analogy with the 

 entire rostrate umbo of the ventral valve in the Rhynchonellae, Terebra- 

 tulse, etc., as a specialized extension of the valve for the protrusion of the 

 pedicle (compare the extreme development of the umbo in the genus 

 Tcrcbrirostra). That these parts are also homologues, it is difficult to prove, 

 on account of the pedicle-sheath becoming more degenerate as maturity 

 approaches, but, assuming this homology, we may regard the sheath and its 

 gradual disappearance, 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, it- 

 self confirmatory of such degeneracy. 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 Strophomena rhomboidalis and Streptorhyn- 

 chus subplanum, while the aperture of the dorsal valve is filling part 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 protruded as much on one side as the other, a fact indi- 

 cative of an extreme lack of differentiation in the two valves in the articu- 



