NO. 6 ECHINOID DISTRIBUTION AND HABITS KIER, GRANT S7 



that this abnormality dates from a stage before any skeletal parts had 

 formed. With ocular IV absent, no plates could be introduced for 

 ambulacrum IV and its two adjacent half-interambulacra for as 

 shown by Jackson (1912, p. 26; 1927, p. 491) new plates are intro- 

 duced at the edges of the ocular plates. This prospective gap was 

 filled by the joining of the anterior half of interambulacrum 3 with 

 the posterior half of interambulacrum 4. Ambulacrum III which is 

 normally anterior shifted to the side of the test with its phyllode 

 entering the side of the peristome instead of the front (pi. 16, fig. 6). 

 The calcareous process that normally occurs in the interior at the 

 edge of the peristome, attached to the first plate of interambulacrum 

 4, also is absent because that plate is missing. It is significant that 

 the echinoid was able to live without this process. We have been able 

 to find little in the literature concerning this structure, but from our 

 study of dissected specimens of Meoma ventricosa, and as figured in 

 the Traite de Zoologie (Grasse, 1948, vol. 11 p. 157, fig. 183) in 

 Spatangns purpureus Muller, the esophagus is attached to this process 

 by numerous mesenteries. Presumably this process keeps the 

 esophagus from being pushed posteriorly by motion of sand through 

 it as the echinoid moves anteriorly. 



One tetramerous variant of Encope michelini also is referrable to 

 Jackson's group 4; it lacks ambulacrum IV and the two associated 

 half -ambulacra. In this specimen (pi. 15, fig. 7) the anterior petal is 

 shifted to the left. Because only four ambulacra occur at the 

 peristome, and the lantern consists of only four jaws, teeth, and other 

 parts, this abnormality also predates the development of any skeletal 

 parts. 



In contrast to the previous two variants, which are probably muta- 

 tional, one specimen of Echinometra lucunter is incompletely tetram- 

 erous, probably because of post-embryonic injury. Adapically (pi. 16, 

 fig. 2), only four ambulacra and four interambulacra are present, 

 with ambulacrum I and its associated half-interambulacra missing. 

 Adorally (pi. 16, fig. 1), all five ambulacra and five interambulacra are 

 present but ambulacrum I and its half-interambulacra (pi. 16, fig. 4) 

 terminate a short distance from the peristome and their place is filled 

 by interambulacral plates from oculars V and II. The lantern is 

 normal with five components of each structure, and there are five 

 auricles. Apparently, therefore, the production of plates for ambu- 

 lacrum I and its half-interambulacra ceased when the echinoid was 

 small and had produced only a few plates in each column. This cessa- 



