INVERTEBRATE PALAEONTOLOGY. 205 



As this rule, however, if systematically carried out, especially in con- 

 chology, would create great confusion in the nomenclature of the older genera, 

 it has not been generally followed. On the contrary, both the British and 

 American associations for the advancement of science, recommend the rule of 

 allowing the first author that follows another who proposed a genus including 

 more than one group, without in any way designating the type, to decide to 

 which of the included forms the original name shall be applied in making 

 subdivisions. The first author, I believe, who followed Linnaeus in the use 

 of the name Mactra, was Midler, in 177G (Prodr. Zool. Dan.); and he men- 

 tions only the one species Mactra solida, Linn., the type of Gray's later group 

 Spisula, generally regarded as generically distinct from Mactra, as now most 

 usually understood. So, if we were to regard him as deciding which is to be 

 taken as the type of Mactra, none of the groups defined in the foregoing 

 diagnoses would belong strictly to that genus. 



The fact, however, that Miiller was merely enumerating a local fauna 

 in his Prodrome, and did not propose to restrict or divide the genus, seems 

 to be a sufficient reason for not regarding him as having selected Mactra 

 solida as the type of the genus Mactra. In 1799, however, Lamarck, who 

 was not making a mere list of a particular fauna, but writing a classification 

 of the Mollusca, cites (in his Prodr., 85) Mactra stultorum, Linn., alone, as 

 the example of the genus, as he also did in his later works. Consequently, 

 Gray, Ilerrmannsen, and many others, have viewed this species as the type; 

 which is also done in the foregoing definition of the same. 



As thus defined, the genus Mactra will be seen to be closely allied, 

 however, to Spisula, which differs chiefly, first, from the Trigonella group, in 

 having its lateral teeth cross-ribbed, and its ligament not separated from the 

 cartilage-pit by a shelly ridge. In the greater length and cross-ribbed 

 character of its lateral teeth, as well as in its more rounded pallial sinus, it 

 also differs from the Schizodesma section. 



A few Jurassic, and some even older species, have been referred to the 

 genus Mactra ; but, so far as I am informed, none of them are known to 

 possess the hinge-characters of this group. From the Cretaceous, however, 

 species showing the hinge and other characters of sections of Mactra are 

 known ; while epiite a number of others, the hinges of which have not been 

 seen, present so exactly the general physiognomy, as well as the muscular 



