CLASSIFICATION OF THE BRACHIOPODA. 91 



The vascular impressions in the dental valve consists of two principal trunks 

 originating on each side between the cardinal and pedicle muscles : these soon divide into 

 two primary branches which extend right and left almost parallel to the margin, and giving 

 off at variable intervals smaller bifurcating veins, which are directed towards the edge of 

 the shell. 



Ohs. Dalman founded the genus atrypa} which he characterized as " Inequivalve 

 biconvex, hinge-line rounded, beak of larger valve covering the base of smaller valve, apex 

 imperforate!' Naming a number of examples,* the first and second being A. reticu- 

 laris of Linn, and aspera of Schl. ; but, unfortunately, these shells happen to have 

 perforation, although the aperture is very often concealed by the curvature of the 

 beak ; the term Atrypa is therefore a misnomer, implying a zoological mistake, and should 

 rightly be expunged; but many authors appear anxious to retain the name, as a simple 

 denomination, casting aside its derivation, and considering alone the^^r^^ or typical species. 

 I have consented (against my own inclination) to adopt the term on the understanding that 

 it will be restricted to those shells possessing the well-defined character of A. reticularis. 

 M. d'Orbigny's name, Spiriyerina, published in 1847, must therefore be considered as a 

 syiionyme.3 



Many important characters distinguish and connect this genus with other sections; thus 

 the beak of Atrypa bears resemblance to that of some Terebratula, being perforated by a 

 circular opening partly surrounded by a deltidium ; but this foramen is not visible in all 

 examples of the same species, from the beak touching and overlying the umbo of the other 

 valve, the animal was therefore probably unattached or free during a portion of its 

 existence. To Bhynchonella, it seems allied by the arrangements of its muscular system, 

 but not by those of its labial appendages, which in the last-named genus were free or 

 affixed only at their origin, while those of Atrypa were supported by large spiral ap- 

 pendages,* so that the section may be said to combine characters belonging to Spirfer, 

 Rhynchonella, and Terebratula, so as to constitute an organisation different from each in 

 some particular. Out of the list of species placed in Atrypa by Dalman, three only can be 



' Petrefacta suecana in the ' Konigl. Vet. Acad. Han dl.' for 1827, published in 1828 (a, privative, 

 and rpwTrn, foramen). 



2 As observed by Professor King, in the 'Ann. and Mag. of Nat. Hist.,' vol. xviii, p. 29, 1846. 

 Dalman included some very different shells in his genus, A. gallata, belonging to Sowerby's Pentamerus, 

 and A. nuceJla to Fischer's Rhynchonella, &c. 



^ In his work on the 'British Pal. Foss. of the Camb. Mus.,' Professor M'Coy adopts M. d'Orbigny's 

 name Spirigerina in preference to Atrypa, but retains his own name Athyris, the derivation of which is 

 quite as objectionable as that of Dalman. 



* The spires of A. reticularis and prunum have been known for several years, and figured by Blainville, 

 V. Buch, and Quenstedt, &c. ; the last-named author, in his ' Handbuch der Petrefackunde,' 1851, proposes 

 to place A. reticularis and prunum in a group termed " TerehratvlcE calcispirce,'" but the shells 

 in question differ so essentially from Terebratula proper, as to require a separate denomination, and to be 

 removed from that family. 



