BRACHIOPODA. Ill 



Shell ornamented with concentric lines and ridges, the epidermal layer 

 bearing hollow spines, which are distended at the base. Shell-substance cal- 

 careo-corneous, the layers beneath the epidermis being punctured by radiating 

 and branching tubules. These layers are concentric and not parallel to the 

 internal surface. 



Type, Siphonotreta unguiculata, Eichwald. 



Observations. The foregoing description, with some essential modifications, 

 is derived from the diagnosis of Siphonotreta, as given by de Verneuil, 

 KuTORGA and Davidson. The term as used by these writers, especially by 

 KuTORGA, has been allowed to include species which differ considerably in the 

 position and structure of that feature of first importance, the pedicle-aperture, 

 and these forms, under a proper conception of congeneric characters, it seems 

 advisable to withdraw from the genus. 



In Kutorga's elaborate study of Siphonotreta and its allied genera,* the 

 position of the siphoiial opening was considered as varying between the cardi- 

 nal margin and the center of the valve. Strictly, however, the opening takes 

 neither of these extreme positions in Siphonotreta, for in no species known is 

 the umbo of the pedicle-valve marginal, and therefore the aperture, which may 

 be regarded as always truncating the beak, can not be marginal. For the spec- 

 ies Siphonotreta tentorium, Kutorga, in which both apex and aperture are at the 

 center of the valve, the author himself suggested the name Mesotreta, and the 

 term may be useful as designating a form intermediate between Acrotreta and 

 Siphonotreta. Under the discussion of the genus Schizambon, reasons are given 

 for provisionally adopting Mr. Walcott's suggestion, that such species as Siphono- 

 treta fissa, Kutorga, in which the external opening of the siphonal tube is situated 

 in front of the apex and connected therewith by a conspicuous groove, should 

 not be regarded as congeneric with S. unguiculata and other forms in which 

 this aperture is circular and apical. Although there is considerable difference 

 in the size and external expression of the type-species of Schizambon (S. typicalis, 

 Walcott,) and the Russian and Canadian representatives of Siphonotreta fissa, yet 



*Ueber die Brachiopoden-Familie der Siphonotretsese, p. 261, pis. vi, vii. 



