370 Certain Shells in the British Museum, 



a much more important difference. But I must be permitted 

 to ask what this paragraph is to prove ? That certain effects 

 are consequent upon the operation of certain causes, every 

 one will readily admit ; but Mr. Stutchbury has not shown 

 that any certain cause has produced any particular effect : he 

 has only supposed the existence of certain causes, which, not 

 having existed, the consequent effect has not been produced. 



I will now conclude, by asking one more question. Mr. 

 Stutchbury has called Mr. Kiener's work " beautiful : " why 

 did he not also term it correct? 



[See also some remarks by Mr. Lewis, p. 387. — EdJ] 



Art. VII. A Synoptical Catalogue of the Species of certain Tribes 

 or Genera of Shells contained in the Collection of the British 

 Museum and the Authors Cabinet ; with Descriptions of the 

 new Species. By John Edward Gray, F.R.S., &c, President 

 of the Botanical Society of London. 



Family MA'CTRADiE Gray. 



The mantle's lobes free on the lower front edge, united before and be- 

 hind, and extended into two united retractile siphons ; the foot lanceolate 

 subanterior. The cartilage internal, placed in a triangular cavity behind 

 the cardinal teeth. Cardinal teeth two in each valve ; the hinder one small, 

 compressed, often rudimentary; the front one triangular, more or less 

 deeply nicked beneath. The lateral teeth of the left valve simple, and 

 fitting between the two on the right valves. The siphonal inflection distinct. 



The genera are easily distinguished by the modifications of the ligament ;. 

 thus : — 



1. Scissode'sma Gray. Ligament external, in a slit. 



2. Ma'ctra Linn. - Ligament external, in a marginal groove. 



Ligament subexternal, marginal, not separated 

 from the cartilage. 



3. Spi'sula Gray - a. Hinder lateral teeth double and single. 



4. Lutra^ria Lam. - b. Hinder lateral teeth single or wanting. 



Ligament internal, in the same cavity as the 

 cartilage. 

 .5. Muli'nia Gray - a. Lateral teeth simple. 

 6. Gna'thodon Gray b. Front lateral tooth hatchet-shaped. 



I. Scissode'sma Gray. 



Shell ovate, trigonal, subangular at each end. Hinge and lateral teeth 

 like Mactra. Siphonal inflection ovate, distinct. Ligament external, in an 

 oblique triangular groove, open into the upper edge of the cartilage pit. 



1. Scissodesma Spengleri { Jig. 29.); Mactra Spengleri Linn. Gmel.y Spengler 

 Cat., t. 3. f. 1—3. ; Chemn., vi. f. 199—201. ; Ency. Meth., t. 252. f. 3.; 

 Sow. Gen., f. 1. 



Inhabits the Cape of Good Hope. B. M. 



2. Scissodesma nitida ; Mactra nitida Schroet. Enl., t. 8. f. 2. ; Mactra co- 

 rallina Chemn. 



Inhabits African seas. British Museum. 



