76 



THE GEOLOGIST. 



grandis, a new species of Trematis, a species of Ambonychia, JBelleropkon sulcatus, 

 Modiolopsis orbicularis, Lingtda 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 Brachiopoda. — Sir, — I should 

 be greatly obliged if some of your correspondents would favour me with a 

 definition of the terms " mesial fold" and " beak ridges," which occur in Mr. 

 Davidson's valuable monograph on the Brachiopoda, published by the Palseon- 

 tographical Society. These two expressions, which are used frequently, are 

 not explained, as I am aware, in the body of the work. — Yours, A Student. 



Our correspondent enquires 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. 



Figs. 1 and 2.—Spirifer. 

 smaller, or dorsal valve ; k, larger, or ventral valve ; a, beak ; h, fissure ; c, pseudo-delti" 

 (hum ; d, area ; e, mesial fold; /, ribs ; s, sinus ; g, lines of growth ; the line m n is the 

 hinge line ; m and n are the cardinal angles, which may be angular or rounded ; the line m 

 h and h n is the cardinal edge ; the margin between o and p is the frontal margin ; between 

 m and o, or n and p the lateral margins. 



The " mesial fold" is therefore a longitudinal mesial elevation which often 

 exists in the dorsal valve of many species of Brachiopoda, and to which corres 

 ponds a sinus in the opposite, or ventral one. The " beak ridges," or "lateral 

 ridges" are ridges which in certain species of Bhynchonella, etc., exist on the 

 lateral portions of the beak, leaving a flattened or concave space between them 

 and the hinge-line. These " lateral ridges" are either continued along the side 

 without recurving to join the hiuge-margin, or, after proceeding some distance, 



