158 DR. T. DAVIDSON OX EECEXT BRACHIOPODA. 



occupied by a lamella in the shape of a double crescent {i and 1-), of which the larger 

 branches (i) partly occupy the large cavities left between the first and second ridges, 

 these being on their inner sides intimately united with the sides of the grooves ; while 

 the shorter branches {^■) are freely suspended over the visceral cavity, and occupy the 

 spaces left between the third and fourth ridges of the ascending apparatus. To these 

 crescent-shaped lamellae M. Deslougchamps has given the name of " descending appa- 

 ratus." The interior of the ventral valve is concave and deep, with a small longitudinal, 

 rounded, mesial elevation ; the hinge-line is straight, and on each side at the base of the 

 deltidium strong hinge-teeth for the articulation of the valves are situated. The beak is 

 hollow ; l)ut there exists on the median line, and far back in the cavity, a small elevated 

 septum, to each side of which is attached a very small thin concave plate (Plate XXIII. 

 fig. 15, b), to Avhich, according to M. Lacaze-Duthiers, the adductor muscle, or " occlusor " 

 of Hancock, was attached. On the bottom of the valve, a little low^er down, and on each 

 side of the mesial elevation, and partly under the cavity of the beak, a large pyriform scar 

 is observable, to which the " divaricator muscles " of Hancock were attached (c) ; and, 

 lastly, external to these, on the bottom of the valve, and near the angles of the hinge-line, 

 there exists a small oval scar (a), which is believed to have been produced by the "ven- 

 tral adjuster " of Hancock. The remaining surface of the shell is closely covered with 

 numerous granulations or asperities. Length 3 lines, breadth 2|, depth 2 lines. 



Hab. Mediterranean, especially on the African coast, in from 30 to 300 fathoms. 

 Bay of Xaples ; off the coast of Sicily. Algerian coast, from Bone to near Cape Eosa. 

 Off Jamaica, 60 to 150 fathoms (L. Barrett). In the Gulf of Mexico (' Blake ' Expedi- 

 tion), 163 fathoms. It is also stated by Sir H. Barkly to occur off Mauritius ; but this 

 statement must be taken with reserve. 



TheckUmn mediterraneum occurs fossil in the Miocene and Pliocene rocks of Italy. 

 Ohs. The animal and its embryology have been admirably described and illustrated by 

 M. Lacaze-Duthiers. We extract the following notes from his important paper : — 



Belative to the muscles, M. Lacaze-Dvithiers states that there are three pairs, six in 

 all, possessing distinct directions and functions. Of these muscles, two (woodcuts, figs. 16, 

 17, 18, a) are lateral, short, wdde, easily observal^le, and serve without doubt for the 

 occlusion of the shell. These are M. Lacaze-Duthiers's " muscles adducteurs lateraux ou 

 externes," which, Mr. Hancock informs me, Avould represent his "adjustors," but that the 

 function of this pair of muscles does not appear to be exactly similar to the " adjustors " 

 of Waklheimia, in which they move the shell upon the pedicle ; in ThecicUum they 

 assist in closing the valves suddenly, and in preventing any irregular or lateral movements 

 W'hich, from the central position of the occlusors, might be liable to take place. In fact, 

 he believes that the function of the "adjustors " in ThecicUum is modified much as it is 

 in the same muscles of Linr/ida; and he is therefore inclined to conjecture that the two 

 valves of the former are not so firmly hinged as they are in the other articulated forms. 

 The second pair of muscles {h of woodcuts) are those nearest to the median line ; these 

 are termed " muscles adducteurs internes " h\ M. Lacaze-Duthiers, and correspond, or are 

 equivalent to, Mr. Hancock's anterior or posterior occlusors. In the central valve these 



