15G DR. T. DAVIDSON ON RECENT BRACHIOPODA. 



directed towards the mouth. This is the rudiment of the spiral portion which is 

 moderately developed in Megerlia, and much more marked in Terehratula. The 

 brachial appendages (in Platydia Davidsoni) resemble therefore those of Platijdia 

 anomioides. The cii'ri of the arms are rather long, .... disposed in pairs, or rather 

 each cirrus seems to he divided into equal filaments. This character has been repre- 

 sented by Mr. Davidson in Platydia anomioides. 



The characters of Tlatijdia Daridsoni are very remarkable. In Megerlia the 

 peduncvilar opening encroaches a little on the dorsal valve, more especially in the 

 variety monstruosa. In JPlatijdia anomioides the foramen is almost entirely on the 

 dorsal valve, and finally in JPlatydia Davidsoni the ventral valve is devoid of fissure, 

 and its beak forms a median projection. This remark, however, is scarcely correct, 

 for there exists under the beak an indication of a shallow fissure. There are, 

 continues Dr. Fischer, important exceptions among the articulated Brachiopoda, since 

 the perforated valve is not foraminated in 'Platydia Davidsoni. All these changes are 

 produced by the shortening of the peduncle, which obliges the Brachiopod to live in close 

 contiguity with a rock, and to rejiroduce the irregularity of the object upon which it is fixed. 

 Since the above was written, M. E. Deslongchamps has described the different stages 

 in the development of the loop in 3Iegerlia truncata, and points out that at one stage 

 the dissimilarity between it and Dlatydia, although apparent, is not so very great. 



In Platydia anomioides the principal stems of tlie loop, after having become attached 

 to the base of the hinge-plate, become fixed to the central pillar-shaped process arising 

 from the bottom of the dorsal valve ; but in Platydia Davidsoni these principal stems, 

 in the specimens examined by M. E. Deslongchamps, Dr. Eischer, and myself, were 

 either broken or absent ; so that to ascertain tlicir real condition more specimens will 

 have to be examined hereafter. 



Family THECIDIID^. 



Genus THECIDIUM, Defrance, 1828. 



We are acquainted with but two recent species of this genus, viz. Thecidium mediter- 

 raneum, Risso, 182(5, and Thecidium Barretti, Woodward, 1861.. The generic characters 

 have been fully described under Thecidium mediterraneum. The animal and embryology 

 of the bst-named species have been admirably described and illustrated by Lacaze- 

 Duthiers and otliers. 



91. Theciditjm mediterraneum, Eisso. (Plate XXIII. figs. 12-22.) 



Theddea mediterranea, Risso, Hist. Nat. des Priiicipales Prod, de TEurope meridionale, t. iv. p. 394, 

 pi. xii. fi-. 183, 1826; Blaiuville, Diet. Sc. Nat. t. liii. p. 434., 1828; Deshayes, Encycl. Meth., Vers, 

 t. iii. p. 135, 1830 ; Pliilippi, Emim. Moll. Sicilife, vol. i. p. 99, pi. vi. fig. 17, 1836, and vol. ii. p. 70, 1844. 



Tliecidea? spondi/lea, ScaeeLi, Cat. Conch. Regni Neapolitani, p. 8, pi. ?, fi^s. 8-10, 1836. 



Tliecidea mediterranea, Dcsh. cd. Lamarek's An. sans Vert. vol. vii. p. 346, 1836; Anton, Verzeichniss 

 Conch, p. 21, 1839 ; Kiister, nov. ed. Chemnitz's Couch.-Cah. pi. 2 c. figs. 22-25, 1843 ; G. B. Sowerby, 

 Thes. Conch, p. 371, pi. ixxiii. figs. 30-32, 1846. 



Tliecidea testudinaria, Michelotti, Dese. des Foss. Jliocenes de PItalie Sept. p. 79, pi. ii. fig. 26, 1847. 



