& W. Ford— Embryonic Forms of Tributes. 267 



forward lobe is considerably larger and spreads out laterally 

 beyond the others, which are of nearly equal shape and dimen- 

 sions. It stands out prominently in the specimen. The eye- 

 lobes are of an elongate semi-lunate form, and, springing from 

 » the postero-lateral angles of the forward lobes of the glabella, 

 extend backward nearly as far as the last glabellar furrow. At 

 the -posterior extremity of each and in direct continuity with 

 them is a well-defined rounded ridge which extends outward 

 and backward toward the postero-lateral margin, with the raised 

 edge of which it unites to form a slender spine. The spaces 

 between the eye-lobes and glabella are tumid and are prolonged 

 backward as two spines lying within the former. These I shall, 

 for convenience, call the " inter-ocular" spines. I have no doubt 

 but that all of these spines terminate in acute points, as they 

 are all seen to taper slightly in the specimen ; but as they pass 

 beneath the stone a short distance from the disk, I have 

 thought it better not to run the risk of destroying them 

 altogether in attempting to further develop them." Between 

 the two spines last described lie the last glabellar lobe and its 

 appendage, the latter a simple, smooth, narrow, nearly flat or 

 only feebly convex plate, convex in outline behind, and giving 

 to the specimen its nearly circular form. This plate I consider 

 to be a rudimentary pygidium. I believe this to be one of the 

 most rudimentary trilobitic forms ever discovered. At present 

 it appears to me to be equally humble in structure with the 

 form representing the first stage of growth of Sao hirsuta illus- 

 trated by Barrande. The head is greatly predominant. There 

 18 5£. tnorax > and no pygidium properly so called. 



With regard to the nature of the spines and ridges which I 

 have just described I became greatly puzzled as my study of 

 the specimen continued. At first I was disposed to look upon 

 them as simply ornamental in their nature, regarding the ridge 

 which runs backward from the eve-lobe as simply the eye-lobe 

 itself fancifully prolonged. Afterward, however, I was led to 

 take the economy of the animal organization into account, and , 

 from the moment of doing this I was conducted to very differ- 

 ent conclusions from those first entertained. I believe the 

 outer spine of this individual to represent the permanent spine 

 of the genal angle in the adult, and the little ridge which 

 j^ns backward from the eye-lobe to this spine and helps to 

 form it to represent that portion of the posterior margin of 

 adults which lies between the genal angle and the facial suture. 

 Ahe mter-ocular spines I am inclined to regard at present 

 as having been most probably protective in their office. Be- 

 tween the ridges extending from the eye-lobes and these spines 

 tne re is an open space or slit in the shield. This space repre- 

 seQ ts, I believe, the position of the posterior portion of the 



