390 MR. H. SAUNDERS ON THE GEOGRAPHICAL 



Fig. 3. Papilla of the mucous membrane of rumen (natural size). 



4. Outline of liver (reduced). 



5. Larynx from behind. 



Plate VII. 



Fig. 6. Male generative organs. The outline of the bladder is seen through 

 the peritoneum. The lettering applies as folio vps : — a a, testicles ; 

 h b, vasa deferentia ; c e, vesiculje seminales ; d d, Oowper's glands ; 

 e, intrapelvic portion of urethra ; /, bulb of urethra ; gg, erectores 

 penis ; h h, retraetores penis ; i, penis ; k, vesicula prostatica ; 

 l.l, ureters. 



7. Intrapelvic portion of the urethra laid open, showing the single 



opening common to the two seminal ducts : a, muscular fibres ; 

 b, erectile tissue. 



8. Glans penis. 



On the Geographical Distribution of the G-ulls and Terns {Laridce). 

 By Howard Saunders, E.L.S., F.Z.S. 



[Read April 18, 1878.] 



To those who hare only a general knowledge of the family of the 

 Laridse, which comprises the subfamilies SternincB (Terns), Bhyn- 

 chopsince (Skimmers), Larince ((xuUs), and StercorariincB (Parasitic 

 Gulls), it may seem that there is but little to be said respecting 

 the geographical distribution of a group whose conditions of exis- 

 tence being almost entirely dependent upon water, and, in the 

 majority, marine, are therefore particularly favourable to disper- 

 sion and general distribution, and opposed to the development of 

 specialized forms. A closer investigation of the subject shows, 

 however, that whereas some members of the family have an ex- 

 ceedingly wide range, there are, on the other hand, many remark- 

 able and isolated forms which, for reasons as yet unknown to us, 

 are restricted to very narrow geographical areas. In some in- 

 stances it is not difficult to trace the connexion with the other 

 members of the same group ; and in other cases the existing gaps 

 between closely allied species may be explained, with a fair show 

 of probability, by the alterations which are known to have taken 

 place in the geographical features of the area now inhabited. But 

 even then it must often be a matter for wonder that birds of such 

 powers of flight should consent (if I may use the word) to remain 

 within such confined limits, when the causes which formerly might 

 have proved a barrier to their extension have for ages disappeared. 



