898 R. T. JACKSON TEACHING PALEONTOLOGY 



The test of a sea-urchin, while superficial, is really an internal skeleton, 

 being covered by living tissue, so that the plates are capable of growth, 

 or resorption, througliout life. Usually, however, the plates have suffered 

 no material change in position or shape during growth, and by their 

 position and number indicate marked stages. Dorsally in Echini, the 

 young, last added plates, both interambulacral and ambulacral, may show 

 youthful characters which, as localized stages in development, can be 

 compared with characters seen in young specimens or in adults of more 

 primitive types. 



In the Pelecypoda, Jackson sliowed that the larval shell, or prodisso- 

 conch, of Ostrea, Avicula, Pecten, and allies, both in the structure of the 

 hard and soft parts, points toward the ancient genus jN'ucula as a radicle. 

 The stages in development of the shell of Avicula and Pecten show a 

 series of changes whicli can be directly compared with the characters of 

 the adults of ancient fossil forms which are allied to and presumably are 

 ancestral forms of these genera. The development of the shell of Perna 

 shows its aviculoid origin in the external characters of the test in the 

 young and also in the development of a series of cartilage pits on the 

 hinge-line from the youthful and also the primitive character of a single 

 cartilage pit. 



The Cephalopoda were the subject of a lifelong study by Professor 

 Hyatt, and on them he based a large part of his philosophical conclusions 

 on biology. In tliis group, stages in development are abundantly shown 

 in the progressive coiling or uncoiling of the shell, all through the nauti- 

 loids and ammonoids. In nautiloids, taking a form wliich has in the 

 adult a close-coiled shell like Coeloceras or Nautilus, we find that the 

 3^oung has first a straight shell for a very brief period, like Orthoceras; 

 then an arcuate shell, like Cyrtoceras ; then an involute stage, like Gyro- 

 ceras; then it becomes continuously more involute until the adult form 

 is attained. In the septal sutures of Ammonites, stages are strongly 

 emphasized, the earliest septa being relatively simple, like early fossil 

 Goniatites, and as one passes from the apex of the cone representing the 

 young to the last built portion of the shell the sutures become progress- 

 ively more complex until full differential characters are attained. In 

 addition to the above, characters of the siphuncle, ornamentation of the 

 test, and characters of the aperture all come in for their share in yielding 

 stages in development to the student. 



In Pelecypoda and Cephalopoda, as also i]i Gastropoda, the shells are 

 for the most part external structures, and, as in the Brachiopoda, youth- 

 ful stages may be studied by following lines of growth of the shell where 

 these have not been destroyed by erosion. This is particularly true of 



