@book {549,
	title = {Pseudothecosomata, gymnosomata and heteropoda (gastropoda)},
	year = {1976},
	pages = {484},
	publisher = {Bohn, Scheltema \& Holkeme Utrecht},
	organization = {Bohn, Scheltema \& Holkeme Utrecht},
	address = {Amsterdam},
	isbn = {9031301760},
	author = {S. Van Der Spoel}
}
@book {554,
	title = {Euthecosomata, a group with remarkable developmental stages (Gastropoda, Pteropoda)},
	year = {1967},
	pages = {375},
	publisher = {J. Noorduyn \& ZN, Gorinchem},
	organization = {J. Noorduyn \& ZN, Gorinchem},
	author = {S. Van Der Spoel}
}
