5. DEVELOPMENT OF BILOBITES* 



(PLATE XXIII) 



THE Linnsean species so well known under the name of 

 Orthis biloba, and so widely distributed in the Silurian rocks 

 of the world, represents one of the very distinct members 

 into which the Orthis group is now divided. It is much 

 removed from ordinary Orthis in general external features, 

 and only by means of developmental characters is it possible 

 to arrive at any idea of its genetic history. 



After having been referred to various genera, including 

 Anomia, Terebratula, Delthyris, and Spirtfer, by different 

 authors prior to 1848, Davidson f first showed conclusively, 

 from a study of the internal characters, that the true rela- 

 tions were with the genus Orthis. Its position has since 

 remained unchallenged, and subsequent investigation has not 

 brought forth any new characters, nor invalidated the results 

 obtained by Davidson. The additional observations here 

 made concerning the development of the shell, while adding 

 to a knowledge of the species, merely serve to bind this 

 form more closely to the group having the broad designa- 

 tion of Orthis. Professor King in 1850 J proposed the genus 

 Dicoelosia for this species, on account of its characteristic 

 form, and authors disposed to divide Orthis have recognized 

 this name. Since then it has been shown that Linne* gave 

 the generic term .Bilobites to the type species of King's genus, 

 and this name is now generally adopted with the rank of a 

 sub-genus. The validity of the specific names applied to 



* Amer. Jour. Sci. (3), XLII, 51-56, pi. i, 1891. 



t Bull. Soc. Geol. France, 2d ser., V, 321, t. 3, fig. 18, 1848. 



J: Monograph Permian Fossils, 106, 1850. 



%v 





