366 Dr. J. E. Gray on some Families of Bivalve Shells. 



tions are not very strong. The embryo has then increased in 

 size, both the intestines and the oesophagus have become longer 

 and thicker, whilst the greater part of the vitelline mass has 

 disappeared. We can discover no more new organs after this 

 period; the remainder of the vitellus entirely disappears, and 

 the embryo shrivels greatly, becomes deformed, and dies. Thus 

 when 2 eggs united, a new organ, the heart, made its appear- 

 ance, although without developing itself perfectly. We have 

 also seen 3 eggs cooperate : the same phsenomena were pre- 

 sented, the only difference being that the organs were more 

 strongly developed, so that the heart became more muscular, 

 and the intestines longer; but as the vitelline mass was ex- 

 hausted before any other organs made their appearance, the 

 embryo began to shrivel, and died. (See PI. XVII. figs. 13-17.) 



[To be continued.] 



XXVII. — A Revision of the Genera of some of the Families of 

 Conchifera or Bivalve Shells. Part III. Arcadse. By John 

 Edward Gray, Ph.D., F.R.S., F.L.S. &c. 



[Continued from vol. xiii. p. 418.] 



Earn. Arcadje, Gray, Ann. & Mag. N. H. xiii. 417. 



Tribe 1. Arcaina. 



The hinge of the more typical form of this family consists of 

 two oblong or linear teeth in each valve, one placed on each side 

 of the line directly under the umbo of the shell. These teeth 

 are divided transversely into cross-ridges, alternating and inter- 

 locking with the cross-ridges of the teeth of the opposite valve. 



The teeth may be compared to the lateral teeth of Spisula in 

 Mactrada and Meretrix in VeneridcBj and more especially to the 

 teeth of the genus Trigonia ; only in this family, instead of the 

 teeth being grooved on the sides, the grooves are sufficiently 

 deep to divide the teeth into transverse interlocking plates. 



The separation of what has been usually regarded as a con- 

 tinuous series of teeth into groups, each forming a distinct tooth, 

 like the teeth of Trigonia, has been overlooked by conchologists, 

 though it was noticed by me in the ' Synopsis of the British 

 Museum,^ in 1840, p. 143, thus : "The hinges of the valve con- 

 sist of a number of transverse interlocking teeth, which appear 

 to be formed by the subdivision oT two elongated lateral teeth.'^ 



The space which separates these two teeth or groups of plates. 



