52 DISTRIBUTION IN TIME AND SPACE. 



tion of the prosobranchiates pass after exclusion from the egg- 

 capsule. Generally, the shell and operculum developed within 

 the capsule are retained by the animal, forming simply the 

 nucleus of the adult structure, but in a few cases it has been dis- 

 covered that a temporary shell and operculum are provided, 

 which are eventually lost. Animals in this larval condition were 

 formerly described as distinct genera of pelagic gasteropods, 

 until Krohn, and after him Macdonald,* showed their true rela- 

 tionship ; in this the lingual dentition became an important agent 

 to indicate the connection with adult forms. Krohn discovered 

 at Messina a curious mollusk which he called Echinospira (pi. 8, 

 figs. 103-105), and which proved to be the larval state of Mar- 

 senia conspicua. He found the nucleus of the permanent shell 

 to be developed within the spiny nautiloid larval shell, and that 

 the latter was eventually cast off. I figure some other pelagic 

 larval mollusks : Macgillivraya, which is the larva of Dolium (pi. 

 8, figs. 99, 100), and Cheletropis = Sinusigera which, on account 

 of its dentition is referred to the Muricidse (pi. 8, figs. 181, 102).f 

 In all the egg-capsules of Muricinae which I have examined the 

 contained shells are miniatures of the adults. Mr. Arthur Adams 

 has referred a Cheletropis to Purpura biserialis, and it is just 

 possible that the species belongs to the Pnrpurina?. Investiga- 

 tions of the transformations of free swimming larvae are made 

 with difficulty, and it will probably be many years before we shall 

 have acquired a sufficient body of facts to understand the condi- 

 tions under which a portion only of the prosobranchiates undergo 

 this larval transformation after exclusion, whilst in most of the 

 genera the newborn mollusk is the epitome of the adult. 



Distribution in Time and Space. 



I shall only recall some main facts here, and that A 7 ery briefly, 

 because the subject is properly larger than my present limits : 

 that is, it can be more advantageously treated with reference to 

 the whole molluscan sub-kingdom. On the other hand, peculiari- 



* Macdonald, On Metamorphosis of Mollusca, Linn. Trans. . xxii, 241 ; 

 xxiii, 69. 



f Chiropteron semilunare, Bars (Beskriv. og Jagtta gelser], t. 14, f. 38, 

 1835), is probably the larva of Aporrhais. Morch Ann. Mag. N. Hist., 

 3d Ser., xvi, 78, 1865. 



