144 C. E. Beecher — Larval forms of Trilobites 



A larval form of Acidaspis. 



Two species of the family Acidaspidas are known from the 

 rocks of the Lower Helderberg group in New York, Acidaspis 

 tuber culatus Conrad, and Dicranurus hamatus of the same 

 author. Both are so different that the larva figured on Plate 

 II, figures 5, 6, 7, can be referred to the first without hesita- 

 tion, since it retains one of the most characteristic features, 

 viz : the denticulate cephalic border. 



Three individuals of Acidaspis tuber cidatus have been found 

 in the same early stage of development. The length of the 

 best specimen exclusive of the spines is -93 mm ; width -88 mm . 

 The total length is l*3 mm . 



The general outline is longitudinally semi-ovate, regularly 

 rounded in front, and truncate behind ; greatest width across 

 the anterior third. Lateral and anterior margins bearing about 

 thirty-five alternating spines or denticulations. Each posterior 

 lateral angle is produced into a strong spine, and between them 

 on the margin are four smaller ones. Upper or dorsal surface 

 convex, highest posterior to the mid-length, and abruptly bent 

 down or truncate behind. 



All but the posterior truncate portion is believed to be 

 homologous with the cephalon of an adult form, so that the 

 posterior angles are really the genal angles. The axis is de- 

 fined, convex, narrow, widest in front, and apparently divided 

 into six annulations or lobes. The posterior one doubtless rep- 

 resents the axis of the pygidium, since it is limited by a fur- 

 row extending to the bases of the genal spines. The next 

 anterior lobe would then correspond to the occipital ring, and 

 the others to the lobes of the glabella. Two lunate promi- 

 nences on each side of the axis in front are the palpebral lobes 

 and the eyes. 



On the ventral side, Plate II, figure 6, the edge of the 

 cephalon is extended inwards, making a continuous doublure. 

 The axial depression extends the entire length of the animal, 

 and is divided into lobes or annulations by transverse ridges, 

 which represent muscular fulcra and points for the attachment 

 of appendages. 



A larval form of Phaethonides. 



The larva figured on Plate II, figures 8, 9, 10, cannot at 

 present be referred to a particular species. Only two species 

 of Phaethonides have yet been described from the Lower 

 Helderberg, but from additional material it is now evident 

 that other species occur in these rocks. Moreover, if this 

 larva retains its specific characters as strongly as Cyphaspis 

 tuber cidatus, it cannot be referred to either Phaethonides 



