the Skeletons of Mollusca, Crustacea and Echinodermata. 383 



shreds with the point of a pen-knife ; this is very easily accom- 

 plished in many fossil species, especially those deeply plicated 

 ones which are usually ranked among the Terebratulce, and best, 

 among recent species, in the Terebratula psittacea. It is by the 

 appearances exhibited by these natural lamincs (fig. 7), that those 

 presented by artificial sections (fig. 6) must be interpreted. This 

 kind of structure is to be seen in Terebratula, Spirifer and Pen- 

 tamerus. In Product a there is a difference in the mode of plication, 

 which takes on the nacreous character ; and it is interesting to 

 remark, that whilst, in the structure of this shell, there is an 

 approach towards the Lamellibranchiata, there is, among the 

 Placunidce (which probably, of all Lamellibranchiata, approach 

 nearest to the Brachiopods), a manifest tendency, in the peculiar 

 lamination of the shell and in the arrangement of the nacreous 

 plications, to the characteristic structure of Terebratula, &c. 



One of the most interesting points in the structure of Tere- 

 bratula, or, at least, in certain species of it, is the existence of a 

 large number of tubular perforations, passing directly from one 

 surface of the shell to the other, and terminating by an orifice at 

 each (fig. 6) . The size of these perforations is sufficiently great to 

 enable them to be detected with a hand magnifier, as minute punc- 

 tations on the surface ; and in this manner I have observed them 

 in all the recent species of Terebratula (about fourteen) which have 

 come under my notice, except in the Ter. psittacea, which departs 

 so widely from the general type, in the incompleteness of the 

 passage for the ligament, that (as Mr. S. Stutchbury has sug- 

 gested to me) it is probably to be considered as the recent type 

 of Spirifer *. Of the very numerous fossil species of Terebratula, 

 I have yet examined but a small proportion ; yet the curious 

 result has uniformly presented itself, that the perforations 

 have invariably been found in the non-plicated or moderately 

 plicated species ; whilst they have been absent in those which differ 

 from the recent species, in being much more deeply plicated. 

 Now the structure of the shell in the genus Spirifer agrees with 

 that of Terebratula in every respect, save the absence of these per- 

 forations f ; I should therefore be inclined, from this character 

 alone, to place Ter. psittacea in the genus Spirifer, and to refer 

 to the same group those deeply-plicated species, which are usually 

 ranked with Terebratula. Whether this be thought a correct 



* I have since learned that this species has been separated, and made the 

 type of a new genus, Atrypa. 



f This statement applies only to the Spirifers of the carboniferous lime- 

 stone ; for in the Spirifer Walcotii of the lias, which I have recently ex- 

 amined, the perforations are present, as in the true TerehratulcB. The whole 

 subject will require very careful investigation before any legitimate conclu- 

 sions can be drawn from this kind of examination, in a group of the value 

 of whose several characters we at present know so little. 



