  For a field $F$ of characteristic zero and an additive subgroup $G$ of $F$, a
Lie algebra $B(G)$ of lock type is defined with basis $\{L_{a,i},c|a \in G,
i>-2\}$ and relations
$[L_{a,i},L_{b,j}]=((i+1)b-(j+1)a)L_{a+b,i+j}+a\d_{a,-b}\d_{i+j,-2}c,
[c,L_{a,i}]=0.$ Given a total order $\succ$ on $G$ compatible with its group
structure, and any $\Lambda\in B(G)_0^*$, a Verma $B(G)$-module
$M(\Lambda,\succ)$ is defined, and the irreducibility of $M(\Lambda,\succ)$ is
completely determined. Furthermore, it is proved that an irreducible highest
weight $B(Z)$-module is quasifinite if and only if it is a proper quotient of a
Verma module.
