DALL : MACTRIDiE AND MESODESMATIDiE. 205 



trifling, rigidly preserved througli long ages of geological time, and 

 their modification accomplished only when the whole organism has 

 submitted to important changes, that one begins to realize that the 

 prominence of a feature is not always the best index to its systematic 

 value, and that there is literally no possibility of knowing any 

 organism too thoroughly. 



In a tyjjical Mactra, well developed and unbroken, the hinge 

 presents the following characteristic armature : ^ a cai-tilage pit or 

 chondrophore ; a group of teeth usually called cardinals, com- 

 prising true teeth, and often accessory lamellae fonned in sinuses of 

 the mantle-edge, but not necessarily belonging to the normal type 

 of dentition ; and the so-called lateral teeth, with laminae in the 

 opposite valve between which the lateral is received. Taken by 

 valves : In the right valve. — An anterior and a posterior pair of lateral 

 laminae; a more or less coalescent pair of cardinal teeth, adjacent 

 above and distant below, forming a A, each arm of which may, or may 

 not, be attended by a delicate accessoiy lamella ]>arallel to the arm. 

 In the left valve. — An anterior and posterior lateral; a single A- shaped 

 cardinal, with or without an accessory lamella on each side of it. 



"When a series of species is compared, it will be seen that the A rotates, 

 so to speak, on its apex, while preserving its apical angle tolerably 

 uniform, in each of the different species. In one the anterior arm 

 may be appressed against the dorsal shell-margin, while the posterior 

 arm is separated from the verge of the chondrophore by a considerable 

 space; in another the posterior arm may project over the cavity of 

 the pit, and a wide sinus appear between the anterior arm and the 

 dorsal margin ; or, again (as in Maotrotoma), the arm of the cardinal, 

 the accessoiy lamella, and one of the lateral laminae may be com- 

 pressed into one radial Kue, so as to present the appearance of a single 

 tooth with three serrations. In typical Mactroids, above the apex of 

 the chondrophore, the anterior end of the Kgament recedes laterally 

 to the point of the umbo. The space between this apex of the 

 ligament and the middle line between the valves is occupied in each 

 valve by a small shelly point, which I call the spur: this is greatly 

 exaggerated in the African Sehizodesma, and in more normal forms is 

 frequently associated with a flat plate which roofs over the apex of 

 the chondrophore. According to the development of the spur and the 

 distance between the beaks of the valves, the diverging anterior ends 

 of the ligament may be far apart, near, or not perceptibly divided. 

 When well marked, the ligament from above has the appearance of 

 a barbed arrow-head, or, as I have named it, is sagittate. Behind 

 and above the cartilage-pit is the scar of attachment of the ligament, 

 walled off in typical Mactroids from the pit by a shelly barrier, which 

 is absent in the Spisuloid forms. I term the pair of projections in the 



1 I prefer to use plain English terms mth a special signification, rather than 

 propose new names of classical derivation, because it is easier for the mind to 

 appreciate new things when it is not simultaneously burdened by having to remember 

 a new vocabulary. 



