606 LINTHIA AUSTRALIS. 



exception of the vertical rows of large primary tubercles, with bare scrobicu- 

 lar areas on each side of the anterior ambulacral groove, above the peri- 

 petalous fasciole. The tubercles increase in size towards the rounded am- 

 bitus, especially at the anterior extremity ; from the ambitus they increase 

 rapidly towards the ambulacra and actinostome, where they are largest and 

 quite distant, owing to the greater size of the bare scrobicular areas. The 

 miliaries are small, closely packed in the intervening spaces between the 

 tubercles, both above and below. The tubercles of the actinal plastron are 

 largest towards the actinostome, decreasing in size near the posterior ex- 

 tremity ; a few large rounded tubercles adjoining the pores near the actino- 

 stome give the phyllodes an unusual degree of prominence. 



In young specimens the outline is more ovoid ; the test is less angular and 

 gibbous than in the older specimens. The posterior amlmlacral (actinal) 

 avenues diverge towards the edge, while, as the specimens become older, they 

 converge somewhat, forming a re-entering curve before tiny trend towards 

 the edge. The posterior petals also simply diverge from the apical system, 

 and, with increasing age, become somewhat curved outward at the extremity : 

 though at no time do we find the least trace of confluence or suppression of 

 the pores in the ahactinal extremity of the posterior zones of the posterior 

 lateral ambulacra ; the petals are invariably well separated by the inter- 

 vening interambulacral space, which forms a rounded keel. 



One of the British Museum specimens is covered with short spines like 

 those of Brissus ; the coloring of the specimen figured by Gray seems anom- 

 alous, as all the other specimens are of a uniform tint, without the mottled 

 coloring of the central part of the plates. A similar pattern of coloration 

 sometimes occurs in M. sternalis, B. carinatus. and B. unicolor. 



