1862.] 273 



parallel with and 3| inches heyond the last caudal Tertebra. This 

 feathered tail is 1 1 inches long and 3| inches hroad, with an obtusely 

 rounded end. This novel and unexpected character of the tail is 

 owing to the constancy with which all known existing and tertiary 

 birds have presented the short bony tail with the terminal modification 

 in most of them of the ploughshare bone* 



Professor Owen next gives the results of investigations into the 

 osteogeny of embryo-birds, showing the number of vertebrae corre- 

 sponding to the anterior caudals in Archeopteryx which coalesce with 

 the pelvis in the course of growth, and the degree to which the 

 posterior caudals retain a resemblance to those of Archeopteryx in 

 the Birds with rudimental wings. From eighteen to twenty caudal 

 vertebrae may be counted in the young Ostrich. In Archeopteryx 

 the embryonal separation persists with such continued growth of 

 the individual caudal vertebrae as is commonly seen in long-tailed 

 Vertebrates, whether Reptilian or Mammalian. The author remarks 

 that the modification and specialization of the terminal bones of the 

 spinal column in modern birds is closely analogous to that which 

 converts the long, slender, many-jointed tail of the modern embryo- 

 fish into that short and deep symmetrical shape, with coalescence 

 of terminal vertebrae into a compressed lamelliform bone, like the 

 ' OS en charrue ' of birds, to which the term ' homocercal ' applies ; 

 such extreme development and transformation usually passing through 

 the heterocercal stage, at which, in palaeozoic and many mesozoic 

 fishes, it was arrested. Thus he discerns in the main differential 

 character of the mesozoic bird a retention of structure which is em- 

 bryonal and transitory in the modern representatives of the class, 

 and consequently a closer adhesion to the general vertebrate type. 



The least equivocal parts of the present fossil declare it to be a 

 Bird, with rare peculiarities indicative of a distinct order in that class. 

 Although the head is absent, the author predicts, by the law of cor- 

 relation, a beak-shaped mouth for the preening of the plumage ; and 

 he also infers a broad and keeled sternum hi correlation with the 

 remains of feathered organs of flight. 



The paper is accompanied by drawings of the fossil and its parts, 

 and of homologous parts in Birds and Pterodactyles. The author as- 

 signs to the fossil animal the name of Archeopteryx macrums. 



X2 



