406 MH. w. p. PTCBAET ON THE [Mar. 21, 



b'. Width of interorbital region of the frontals exceeding I- that of the 

 frontals between the lachrymals ; pterygoid ends of the palatines gently 

 widening from behind forwards Thalassogeron. 



b, Interorbital region of frontals reduced to a narrow median ridge ; antorbital 

 plate extending outwards to the level of or beyond the outer border of the 

 lachrymal ; no tubercle between the mammillary processes. Phoebetria. 



B. Vbetebe,^. 



All the presynsaeral vertebrae are free and heteroccelous ; the centra of the 

 thoracic bear more or less conspicuous lateral depressions and are often highly 

 pneumatic ; only the 2nd-5th or 6th cervicals bear neural spines ; all the cervi- 

 cals from the 2nd bear ribs iu the form of elongated and very slender styles, 

 they become free and bear a distinct capitulum and tuberculum on the last 3 

 vertebrae (cervico-dorsals) ; the free caudal vertebi-a2 bear distinct interceiitra, 

 and in the larger species the neural arch of each is provided with a pair of 

 processes directed forwards and embracing the neural spine of the vertebra 

 next in front ; catapophyses of cervicals never meeting in mid-ventral line to 

 form a canal. 



A. Many if not all the thoracic vertebrae bear elongated hypapophyses, of 



which the cephalad are bifid PrucellM-iidce. 



B. Thoracic vertebrae without hypapophyses Diomedeidcs. 



C. Sternum and Peotoeal Girdle. 



The posterior border of the sternum maybe either notched or entire. In the 

 former the anterior coracoid border is produced forwards beyond the level of 

 the anterior lateral process. In the latter the anterior coracoid border does 

 not project far forwards. The base of the coracoid is always of great width, 

 and the"furculum articulates by ligament with the an tero- ventral angle of the 

 carina. 



A. Pneumatic foramina of the dorsal surface of the sternum, when present, 

 never very conspicuous, and confined to the anterior region of the middle 

 line ; carina sharply defined throughout the whole length of the sternal 

 plate; spina interna absent Procellariklce. 



a. Coracoid grooves forming markedly oblique angles with the long axis of 

 the sternum ; base of coracoid more than ^ as broad as long ; articular 

 surfaces of sternal ribs extending backwards far beyond the anterior 

 lateral processes, 

 a'. Posterior border of the sternum entire ; clavicle witii a hypocleideum ; 

 spina externa pointed, 

 a". Hypocleideum small ; width of posterior border of sternum much less 



than its long axis Procellaria. 



b". Hypocleideum long. 



a?. Width of posterior border of sternum = length of long axis of sternal 



plate ; carina unf enestrated Cymodroma. 



b^. Carina fenestrated ; posterior border of sternum wider than length 



of long axis Fregetta. 



c'. Carina fenestrated ; with posterior border of sternum less than long 

 axis ; antei-ior lateral process only slightly projecting above the 

 base of the coracoid ; width of base of coracoid falling far short 



of length of its long axis Oceanites. 



d^. Metasteruum projecting beyond the posterior lateral processes; 

 carina fenestrated ; anterior lateral processes projecting con- 

 siderably over the base of the coracoid ; width of coracoid at base 

 nearly equal to the length of the shaft ; width across posterior 

 border of sternum less than its long axis Pekiffodroma, 



