656 THE SPINES. 



of the actinal plastron, are spoon-shaped at the tip (PL XXXVII. f. is). In 

 the Clypeastridae the spines are frequently serrated (PL XXXVII. f. i, 4) 

 much as the spines of the Desmosticha. The spines of the Petalosticha are 

 generally smooth (PI. XXXVII. f. 18,20). In some of the longer curved 

 spines of the abactinal surface prominent distant serrations are developed, 

 ns in Maretia (PL XXXVII f. i>,) and Lovenia (PI XXXVII f. 17). 



In transverse sections of the spines of Scutellidae the interior is hollow, 

 with large well-separated marginal wedges, and but little cellular interla- 

 mellar substance (PL XXXVII f. 5, 6, 8), rendering the spines of the Scutel- 

 lidae quite transparent. The spines of Echinanthidae (PI. XXXVII f. 9, 10) 

 resemble quite closely those of the Spatangina (PL XXXVII f. is, 15), in 

 which we have a large hollow central space, with two or three concentric 

 rings of cellular substance intercalated as narrow strips between the solid 

 marginal wedges. The structure of the spines of the Scutellidae is quite 

 similar to that of the Arbaciadae, if the latter had a hollow central shaft. 



