from the Lower Helderberg Group. 147 



Nepionic 

 (As many stages as 

 normal thoracic 

 segments.) 



Cephalon distinct. 



Thorax incomplete. 



Pygidium distinct. 



Examples : Agnostus, Microdiscus, and 

 young of all trilobites lacking full num- 

 ber of thoracic segments. 

 f Cephalon distinct, complete. 



■xt • j Thorax distinct, complete. 

 JNeanic < -r> •-■• -.. „.. ' r -, . 



j Fygidmm distinct, complete. 



[_ Growth incomplete. 



p , i . j All parts complete, and full 



size attained. 



It is interesting to note that while neither the larvse of 

 Acidaspis nor Phaethonides possess the characteristic generic 

 features of sutures and glabellar lobation, yet their specific 

 facies is quite pronounced. The larval Acidaspis does not 

 suggest Acidasjns nor Ceratocephala in general ; as Cerato- 

 cephala Verneuili JBarr., Solenopeltis Buchi Barr., Dicranurus 

 hamatus Con., or Ancyropyge Romingeri Hall, but has mani- 

 fest likenesses to its own species, Acidaspis tuberctdatus, and 

 other closely allied forms, as A. Brighti Murchison, A. 

 Hoernesi Barr. and also several species of Odontopleura, which 

 in their adult form resemble the species of Acidaspis in every 

 way except in the absence of the occipital spine. 



Tale Museum, New Haven, Conn., June 10th, 1898. 



EXPLANATION OF PLATE II. 

 Acidaspis tuberculatus Con. 



Figure 1. — Cephalon of an adult specimen, x 2. 

 Figure 2. — Pygidium of a mature individual, x 2. 

 Figure 3. — Pygidium of an immature individual, x 5. 

 Figure 4. — Segment of thorax ; showing spines and ornamentation, x 2. 

 Figure 5. — Dorsal view of larva, x 38. 

 Figure 6. — The same; ventral view, x 38. 

 Figure 7. — The same; profile, slightly oblique, x 38. 

 Lower Helderberg group, Albany Co , N. Y. 



Phaethonides, sp. 



Figure 8. — Dorsal view of larva, x 38. 

 Figure 9. — The same; ventral view, x 38. 

 Figure 10.— The same : profile, slightly oblique, x 38. 

 Lower Helderberg group, Albany Co., N. Y. 



