DR. T. DAVIDSON OX RECENT BRACHIOPODA. 121 



examples of Kyaussina rubra, and consequently can say nothing with respect to the 

 modifications through which the young shell passes previous to attaining its fullest 

 development, but that the disposition of this appendage in young K. Davidsoni establishes 

 points of transition so complete between the genera 3Iefferlia and K^^missina tliat there 

 is not the slightest doubt in his mind as to the position of these genera in the zoological 

 series, and that they certainly belong to one group. The analogy becomes apparent 

 when the details of the adult Kranss'ma Lamarckiana and the transitory state of Megerlla 

 trimcata are considered. 



I have never seen any very young examples of K. rubra ; the smallest I have examined 

 measured 4 lines in length, the largest 12, and no material difference could be perceived 

 in tbe forked deviating lamellae. I do not see that 3Icgerlia trnncata and Kraussina 

 rubra are more nearly related than are Terebratella, Laquens, and several others. 

 We must value the characters of the genus or subgenus and species from the adult and 

 full-grown condition, and not from that of the early stages of growth ; for many 

 specimens prior to attaining their adult condition and characters pass through several 

 metamorphoses or modifications in shape, as has been so admirably demonstrated by 

 Herman Eriele, E. Deslongchamps, and others. There are also notable differences 

 between Megerlia and Kraussina in the shape of the hinge-plate, muscular scars, and 

 labial appendages, which must be taken into consideration. 



70. Kraussina cognata, Sowerby*, sp. (Plate XX. figs. 24-26, var. ? 27-30.) 



Terebratula cognata, Sowerby (not of Chemnitz) , Thes. Conch, i. p. 346, pi. 68. figs. 12-1 1, 1846. 

 Kraussia cognata, Davidson, Ann. & Mag. Nat. Hist. 2nd ser. vol. i.x. p. 370, 1852. 

 Terebratula {Kraussia) cognata, L. Reeve, Conch. Icon. pi. is. fig. 38, o, 6, 1861. 

 Kraussina cognata, Dall, Amer. Joum. of Conch, vi. p. 140, 1870. 



Shell somewhat sulitrapezoidal or elongated, semicircular ; hinge-line nearly straight, 

 almost as long as the width of the shell, with slightly rounded or angular extremities. 

 Dorsal valve semicircular, rounded in front, with or without a slight longitudinal median 

 depression along the anterior half of the valve. Ventral valve deep, and much more 

 convex than the dorsal one. Beak short, very little incurved, foramen large and incom- 

 plete, margined anteriorly by the umbo of the dorsal valve, and by two small lateral 

 deltidial plates ; beak-ridges sharply defined, leaving between them and the hinge-line 

 a triangular area. Surface of the valves radially marked by fine radiating ribs, increasing 

 in number at various distances from the beak and umbo by the interpolation of shorter 

 riblets, which are more sharply defined in some specimens than in others. Colour 



* [No one can tell from Chemnitz's description and figures what reaUy constitutes his Cocjnata Anomue craniohris 

 basi perforata (Neues syst. Conchylien-Cabinet, vol. viii. p. 78, tab. 76. fig. 088 a, b, 1785). That author's unsatis- 

 factory description and illustrations in no way resemble those of the specimen described and figured by G. B. Sowerby 

 in his 'Thesaurus Conchyhorum ' (vol. i. p. 346, pi. 68. figs. 12-14, 1846). The Anomia cognata of Sowerby (not 

 of Chemnitz) is now in the British Museum. Chemnitz's figure represents a smooth shell, ovate in form, and with a 

 rounded hinge-lino. Sowerby 's T. lorjnata is squarely suborbicular, with a long, nearly straight, hinge-line, the 

 external surface being covered with numerous small radiating ribs. Chemnitz's species is unidentifiable and should 

 therefore be expunged, and the TerehraUda mrjnata, Sowerby, retained as the type of the species. — T. D.] 



