250 Mr. T. Davidson^s Notes and DesaHptions 



names have also been proposed for these plaited Terehratula, such 

 as HypothyriSy Phillips, Cyclothyris (M^Coy), Atrypa (M^Coy)*, 

 Hemithiris'f and Acanthothiris, D'Orb., &c. 



Without entering into a full history of this genua, our 

 present object is, principally to inquire if authors are really 

 entitled to establish these different genera ; if the organizations 

 vary ; and if the genus existed from the Silurian period up to 

 the present day ; points which require immediate investigation 

 to prevent the confusion which is daily added to the nomencla- 

 ture by authors adopting frequently at random one denomination 

 instead of another. Are we entitled to follow M. D^Orbigny, who 

 places, for example, 



T. psittacea. 1 • i • tt 



T.Wilsoni, kc. |m his genus Hemithieis; 



T. octoplicata, 1 



T, concinna, >in Fischer's genus Rhynchonella ; 



T. tetrahedray &c. J 



T. spinosa, \ • x.- a o 



T. Lticosa, &c. /'" '^•^ S^"''^ Acanthothiris ? 



On what principle is a genus among the Brachiopoda to be 

 established ? Is it merely from a slight dissimilarity of outward 

 form, or is it from the important differences and peculiarities in 

 the supports of the arms and other impressions left by the animal 

 in the shell ? The great value of these last was justly admitted 

 by M. D'Orbigny, but not always attended to by that author. 



The genus Rhynchonella may be characterized as follows : — 



existence of which I was first informed by Mr. Morris, who had discovered 

 the only copy in London in the library of the Linnajan Society ; but Prof. 

 King was the first to publish it, having also discovered the paper. 



For more ample details on this subject I beg to refer to my work on 

 British Brachiopoda, Part 1, 2 and 3, genus Rhynchonella (Palaeontogra- 

 phical Society). 



* The genus Atrypa was estabhshed by Dalman in 1827, his types being 

 A. reticularis, A. aspera, A. galeata, A. nucella, A. crassicosta, A. lenticu- 

 laris. This unfortunate so-called genus has of late been the subject of much 

 discussion and variety of opinion among several authors, but on which 

 our space will not allow us to enlarge at present; suffice it to say, that the 

 three first-mentioned shells are well-known, and do not possess the cha- 

 racteristics of those now placed in the genus Atrypa both by Prof. M'Coy 

 and M. D'Orbigny, many of which I am convinced are only Rhynchonellce. 



t We do not wish it to be understood, that we consider all the species 

 placed by authors in the above-enumerated genera to belong to one single 

 section, or that all the species placed in M. D'Orbigny's genus Hemithiris 

 are true Rhynchonella, only those (among which are his types) partaking 

 of the characters visible in R. psittacea, Wilsoni, &c. M. D'Orbigny places 

 in his genus Hemithiris certain forms, such as T. Schlotheimii, which we 

 believe to fonn a separate section, to which Prof. King has given the najme 

 garner ophoria. 



