52 BRITISH CARBONIFEROUS BRA.CHIOPODA. 



opinion founded upon the minute inspection and comparison of two hundred or more 

 indivichials. In his work on ' British Palaeozoic Fossils,' Professor M'Coy advocates the 

 separation, and states that in Sp. rotundata, Sowerby {Sub-rotundata, M'Coy), " the 

 depression of the sides or sharpness of the margins form the acute angles at which the 

 valves meet each other, less gibbosity and greater proportional width distinguish specimens 

 of all sizes from the little Sj). pinguis of Sowerby, with which in other characters it is 

 identical." Therefore, according to Professor M'Coy, both are identical in all their 

 characters, excepting those of size, gibbosity, and the angles at which the valves meet each 

 other. Any one, however, who has studied a number of specimens will soon feel convinced 

 that dimensions, gibbosity, and angles at which the valves meet cannot be considered of 

 specific importance in this present case, since these characters vary in almost every individual, 

 and that every link may be traced connecting the most extreme variations presented by 

 either forms. In Plate X, figs. 1—7 represent Sowerby's Sp. pinguis, figs. 8 — 12 his 

 Sp. rotundntus ; but the first is indubitably either the young of that author's rotundatus, 

 or aged specimens stinted in their regular development, and which is due, no doubt, to 

 local or accidental circumstances, the shell having acquired greater thickness or depth in 

 proportion to its length and width. Therefore, since all the other and more impori^nt 

 characters are identical, we cannot do better than to follow Mr. Morris,^ while retaining 

 but one name {Sp. pinguis) for both of Sowerby's species.^ It should likewise be observed 

 that on many full-grown specimens of S. rotundata can be seen interruptions in develop- 

 ment which agree exactly with the form known as Sp. pinguis, but which, on the shell 

 resuming its growth, assumed the shape and proportions of Sp. rotundata, Sow. Some- 

 times also during the earlier portion of their development the ribs were all regular and 

 simple, when, after a sudden interruption, a certain number suddenly bifurcated, as will 

 be perceived in fig. 8. In some examples the ribs are regular and simple on one half of 

 the valve, while on the other several became bifurcated or irregular in their respective 

 widths. 



In all well-grown specimens that have come under my observation, the length of the 

 hinge line has been rather shorter than the greatest width of the shell ; still, in a few 

 dwarfed individuals, such as in fig. 5, the hinge line is as long as the greatest width of 

 the shell, and the cardinal angles are not rounded ; this and other exceptional appearances 

 cannot be taken as the normal condition of the shell, and require to be viewed in the light 

 of those malformations to which all species in the animal or vegetable creation are more or 

 less subjected. ^ 



^ ' Catalogue,' p. 153. 



^ The laws of priority oblige us to make use of the term pin<juis, that of rotundata having been 

 applied to another species. 



^ Geometrical measurements have been resorted to in their descriptions by several naturalists, such as 

 Professor M'Coy, Mr. Kocklin Schlumbtrgcr, and others, but the apical or other angles in a Brachiopod 

 are so exceedingly variable in different specimens of a same species thai they become of no value whatso- 



