76 
THE GEOLOGIST. 
grandh, a new species of Trematis, a species of AmhonycUa, Bellerophon sulcatns, 
Modiolopsis orhicidarh, Lingnla ovata, and a_ species of Nucula. 
The reader is referred for further information on this interesting and beautiful 
locality to Messrs. Salter and Avelin's paper, in the tenth volume of the 
Quarterly Journal of the Geological Society. — A. ^Marston, Ludlow. 
Terms used in the Description of the Beachiopoda. — Sir, — I should 
be greatly obliged if some of your correspondents would favour me with a 
definition of the tenns " mesial fold" and " beak ridges," which occur in Mr. 
Davidson's valuable monograph on the Brachiopoda, published by the Palaeon- 
tographical Society. These two expressions, which are used frequently, are 
not explained, as I am aware, m the body of the work. — Yours, A Student. 
Our correspondent enquii'es what is meant by the term " mesial fold" and 
" beak ridges ;" perhaps the accompanying sketches will explain these and 
other terms better than could be expressed by words. 
i~ 
Figs. 1 and 2.—Spirifer. 
''^^^^^d''LeS^^fZfJ!vh]^l^\'''' ventral valve; a, beak; 5, fissure; c. pseudo-delti- 
Mn-e'w 1 ^rfn ' f' 9, Imes of growth; the hne m « is the 
Aand K is 7hffJkl^^^ '''''''}^^ ^^^^^ '^^^^ ^^l^r or rounded ; the line m 
m and' ror f ^d^^SeVa^S i'arS?" ° ^ ^^^^^ ^ 
The "mesial fold" is therefore a longitudinal mesial elevation which often 
exists m the dorsal valve of many species of Brachiopoda, and to which corres 
ponds a sinus m the opposite, or ventral one. The "beak ridges," or "lateral 
Wp'fl ^"^..^'^^S^s.^^ic^h in certain species of BhynchoneUa, etc., exist on the 
lateial portions of the beak, leaving a flattened or concave space between them 
InTf These "lateral ridges" are either continued along the side 
without recurvmg to join the hinge-margin, or, after proceeding some distance, 
