170 



LO WER IN VEH TEBllA TES. 



H. niammilatus has ten to eleveu pairs of pores iu eachi arc, while H. tric/onarius has 

 fifteen to seventeen pairs. The spines of the former are nsually stout and bat-shaped, 

 and in color vary from uniform ash-gray or light brown, with white wings at the end, 

 to nearly black. In H. trigouarius tlie spines are usually longer, tapering, and more 

 or less triangular, but Agassiz states that they vary so much that the two species 

 cannot be distinguished by the spines. When a spine of H. manimilatus is broken 

 off at the base, it is replaced by a long tapering triangular si)ine like that of the other 

 species. 



The genus to which the cumbrous name of Strongi/loceutrotus has been given con- 

 tains species with a circular or pentagonal, slightly depressed test, with pores arranged 

 in arcs of at least four or five pairs. There are several species, of which the best 



known is the «S'. drobachiensis of the 

 north-eastern coast of North America, 

 and of Alaska. S. mexicauus occurs 

 ni the Gulf of California, and the test 

 reaches a diameter of nearly three 

 inches ; but these dimensions are far 

 behind those of /S. franciscanus, of 

 the west coast of the United States. 

 In this form the test alone is five or 

 six inches across, and the s))ines are 

 large, so that fine examples measure a 

 foot. In the Echinid* proper, a grouji 

 which contains several genera and 

 species, the test is often nearly globu- 

 lar, — in Amblypneustes the height 

 equals the width. Echiiins esculeiUiis 

 is one of the best known forms, and is found on the coasts of Norway and of England. 

 The test is of a brownish or brick red color. As its name implies, it is occasionally 

 used as food. Ilippono'e depressa is a large sjiecics from the western coast of Mexico 

 and the Gulf of California. 



J'rionechinus sagittif/er, a species found by the ' Challenger ' ex]iedition at depths 

 varying from seven hundred to one thousand and seventy fathoms in the seas around 

 Australia and the East Indian islands, is remarkable from the presence upon the sj)ines 

 of serrations resembling those of Salenia varispi/m, instead of the regular fluting 

 characteristic of most Echinida-. 



Order II. — CLYPEASTRID./E. 



In this order the mouth is placed as in the regular sea-urchins, but the anal open- 

 ing occupies a position innnediately opposite to the odd ambulacrum, and often on 

 the under side. The genital pores retain their position at the sunnnit of the upper 

 surface of the test, which is exceedingly depressed, with its edge or ambitus more t>r 

 less sharp, so that the uj)per and under surfaces are entirely cut off from each other, 

 and are of quite different character. The rows of pores for the exit of the suckeis 

 do not extend around this sharp edge, but form five pairs of curves, arranged some- 

 what like the petals of a flower, upon the upper surface only; while on the oral 

 surface the ambulacra are marked by furrows that converge toward the mouth. The 



